NMFS has determined that the northeastern stock of offshore spotted dolphin is below its maximum net productivity level (MNPL) and, therefore, is depleted as defined by the Marine Mammal Protection Act (MMPA). This determination is based on a review of the best available information.
RULES and REGULATIONS
DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
50 CFR Part 216
(Docket No. 920544-3284, I.D. 102093F)
Taking and Importing of Marine Mammals; Listing of the Northeastern Offshore Spotted Dolphin as Depleted
Monday, November 1, 1993
AGENCY: National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA), Commerce.
ACTION: Final rule.
SUMMARY: NMFS has determined that the northeastern stock of offshore spotted dolphin is below its maximum net productivity level (MNPL) and, therefore, is depleted as defined by the Marine Mammal Protection Act (MMPA). This determination is based on a review of the best available information.
EFFECTIVE DATE: November 1, 1993.
FOR FURTHER INFORMATION CONTACT:
Micheal Payne, Office of Protected Resources, 1335 East-West Highway, Silver Spring, MD 20910, (301) 713-2322.
SUPPLEMENTARY INFORMATION: Section 3(1) of the MMPA (16 U.S.C. 1362(1)) defines the term "depletion" or "depleted" as meaning any case in which the Secretary, after consultation with the Marine Mammal Commission (MMC) and the Committee of Scientific Advisors on Marine Mammals * * *, determines that a species or population stock is below its optimum sustainable population; a State to which authority for the conservation and management of a species or population stock is transferred * * *, determines that such species or stock is below its optimum sustainable population; or a species or population stock is listed as an endangered species or a threatened species under the Endangered Species Act of 1973.
Section 3(8) of the MMPA (16 U.S.C. 1362(8)) defines optimum sustainable population (OSP) as; with respect to any population stock, the number of animals which will result in the maximum productivity of the population or the species, keeping in mind the carrying capacity of the habitat and the health of the ecosystem of which they form a constituent element.
NMFS regulations at 50 CFR 216.3 clarify the definition of OSP as a population size which falls within a range from the population level of a given species or stock which is the largest supportable within the ecosystem (K) to the population level that results in maximum net productivity (MNPL). Maximum net productivity is the greatest net annual increment in population numbers or biomass resulting from additions to the population due to reproduction and/or growth less losses due to natural mortality.
Section 2 of the MMPA (13 U.S.C. 1361) states that marine mammal species, populations and/or stocks should not be permitted to fall below their OSP level. Historically, MNPL has been expressed as a range of values (generally 50-70 percent of K) determined theoretically by estimating what size stock in relation to the original stock size will produce the maximum net increase in population (42 FR 12010, Mar. 1, 1977). In 1977, the midpoint of this range (60 percent) was used to determine if a stock was depleted (42 FR 64548, Dec. 27, 1997). The 60 percent value was supported in the final rule governing the taking of marine mammals incidential to commercial fishing operations (45 FR 72178, Oct. 31, 1980).
Discussion
Background
On October 29, 1991, NMFS was petitioned to designate the northern stock of the offshore dolphin (hereafter referred to as the northern offshore spotted dolphin) as depleted under the MMPA. NMFS published a notification of receipt of this petition, a determination that this petition presented substantial information indicating that the petitioned action may be warranted, and requested comments (56 FR 56502, Nov. 5, 1991). The comment period on the petition closed January 17, 1992. Following a review of the comments that were forwarded to NMFS, and available information, NMFS published a proposed rule to designate the northern offshore spotted dolphin as depleted on June 18, 1992 (57 FR 27207) and set a deadline of August 17, 1992, for comments on the proposed rule.
Comments and Responses on the Notification of Receipt of Petition and the Proposed Rule To List the Northern Offshore Spotted Dolphin as Depleted Under the MMPA
More comments were received by NMFS following the December 18, 1991, notice of receipt of petition (56 FR 65724) than following the proposed listing of the northern offshore spotted dolphin as depleted (57 FR 27207, June 18, 1992). Many of the comments received by NMFS were not specifically directed at the northern offshore spotted dolphin but, rather, were addressed at NMFS survey methodology, the quality of the data on Eastern Tropical Pacific (ETP) dolphin mortality and fishery effort used in ETP dolphin status reviews, and analytical methods used by NMFS to determine the current status of the ETP dolphin stocks relative to historical levels. Specific questions on these subjects and that were provided to NMFS as comments, and responses to these questions by NMFS, have been discussed previously at annual status of ETP dolphin stocks meetings, in the minutes of the Workshop on the Status of Porpoise Stocks (Workshop) following the Monitoring of Porpoise Stocks (MOPS) surveys, 1985-1990 (at DeMaster and Sisson, 1992), and published in the Federal Register as part of the final determination not to list the eastern spinner dolphin, S. longirostris orientalis, as threatened under the Endangered Species Act (57 FR 47620, Oct. 19, 1992), and in the final determination to list the eastern spinner dolphin as depleted under the MMPA (58 FR 45066, Aug. 26, 1993). As a result, they will not be further discussed in this final determination.
One comment, however, has not been addressed in previous Federal Register notices. The Inter-American Tropical Tuna Commission (IATTC) stated that information presented at the November, 1991, Workshop (at DeMaster and Sisson, 1992) did not support the distinction between a northern and a southern stock of offshore spotted dolphins. Perrin et al. (in DeMaster and Sisson, 1992) suggested that the best available evidence was consistent with the existence of a northeastern and a southern-western stock of spotted dolphin, rather than the previously accepted northern and southern offshore stocks. Workshop participants, therefore, recommended new stock boundaries to be used for management purposes.
The IATTC further stated that, given that the scientific evidence did not support the validity of the northern offshore stock, new analyses should be carried out before any conclusions are reached regarding the status of the stocks of the offshore spotted dolphins, and requested that the comment period be extended for six months to allow:
(1) Indices of abundance and incidental mortality based on the new boundaries of spotted dolphin stocks to be recomputated;
(2) Analyses assuming current equilibrium to be conducted; and
(3) New analyses of mortality estimates to be performed.
NMFS believed that, at the time of the proposed rule to designate the northern offshore spotted dolphin as depleted, the best available information accepted by the scientific community indicated that the northern offshore spotted dolphin stock was at a level significantly below OSP. However, the results of the November, 1991, Workshop reviewed scientific information on these stocks and discussed, among other items, possible changes in the structure of ETP dolphin stocks, including offshore spotted dolphins.
Prior to this workshop, and at the time of the proposed rule, ETP spotted dolphins were partitioned into three subspecies from the Eastern and Central Pacific: (1) The coastal spotted; (2) the offshore spotted dolphin; and (3) the Hawaiian spotted dolphin (Perrin, 1975). Evidence for reproductive isolation (Barlow, 1984; Perrin, Coe and Zweifel, 1976; Hohn, Chivers and Barlow, 1985), and morphological differences (Perrin, Sloan and Henderson, 1979; Perrin et al., 1985, 1991) further justified dividing the offshore spotted dolphin into northern and southern stocks. However, two studies that were reviewed at the Workshop (Dizon, Perrin and Akin (1992) and Perrin et al. (1991)) established two new geographical stocks of offshore spotted dolphins based on a re- examination of cranial morphology, the northeastern and western/southern (Fig. 1), making the northern offshore stock definition obsolete (review at Perrin et al., in press). Based on these studies NMFS recommended changes in the stock structure for spotted dolphins in the ETP. The changes which were undergoing peer review at the time of the June 18, 1992, proposed rule to list the northern offshore spotted dolphin as depleted under the MMPA, are as follows:
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Existing stock structure at the time of the proposed rule New stock structure
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Northern .................................................. Northeastern.
Southern .................................................. Western/Southern.
Coastal ................................................... Coastal.
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The proposed rule addressed the status (abundance and fishery-induced mortality) of the northern offshore spotted dolphin using previously accepted stock structure and geographic boundaries, and not the currently accepted boundaries for the northeastern offshore spotted dolphin (hereafter referred to as northeastern spotted dolphin).
Section 115(a)(3)(E) of the MMPA (16 U.S.C. 1383b(3)(E)) provides that if the Secretary finds with respect to such a proposed rule that there is substantial disagreement regarding the sufficiency or accuracy of the available information relevant to a status determination, the Secretary may delay the issuance of a final rule for a period of not more than six months for purposes of soliciting additional information.
Due to the new information indicating that the geographic boundaries which delineate the northern offshore spotted dolphin stock should be revised, and pursuant to section 115(a)(3)(E) of the MMPA, NMFS delayed issuance of a final determination on the proposed rule in order to review the new recommendations, to assess the status of the northeastern spotted dolphin stock, and to solicit additional comments on the northeastern spotted dolphin stock (57 FR 40168, Sept. 2, 1992).
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Comments Received on the Reconfiguration of Stocks and the Northeastern Spotted Dolphin
NMFS received one comment and one set of analyses in response to the request for additional information on the reconfigured stock prior to the issuance of a final determination.
Comment: One commenter hoped that NMFS was not using the redefinition of stock boundaries as a taxonomic manipulation to avoid the need to list. The commenter urged that NMFS work with the IATTC to bring this listing to a prompt resolution.
Response: NMFS has not attempted to use the results of scientific studies on ETP dolphins to avoid its responsibilities under the MMPA. The scientific process that leads to peer-reviewed manuscripts such as Dizon, Perrin and Akin (1992) and Perrin et al., (1991), often takes significant periods of time for analyses, peer review and discussion, manuscript review and publication. The timing of the release of these studies, and the NMFS recommended changes in the stock structure for spotted dolphins in the ETP, was coincident to, but not precipitated by, the receipt of the petition to list the northern offshore spotted dolphin as depleted under the MMPA.
Comment: The IATTC provided comments, and several analyses that examined the status of the northeastern spotted dolphin. The approach used by the IATTC differed from traditional analyses that compare current population size to the historical population size for two reasons: (1) Due to reservations in the ability of the model used in the back-calculation to reflect changes in the population accurately; and (2) due to problems with the mortality data for the 1959-1972 period, and the ability to obtain valid estimates of total incidental mortality for the fishery from these data, at least during the six month time period provided for comments.
Alternative approaches considered by the IATTC included using the series of relative abundance indices, estimated from the data obtained by observers on board tuna vessels (estimation procedures described in Buckland and Anganuzzi (1988) and Anganuzzi and Buckland (1989)), to assess the probability that the population of the northeastern offshore spotted dolphin had declined by more than 40 percent over the period 1975-1991. If the probability were large, then it can be concluded that the population is likely below the MNPL (i.e., below 60 percent of K), therefore depleted under the MMPA. The results from this analysis indicated that, while it seemed that the population has declined since 1975, the evidence suggests that the decline has been less than 40 percent. The IATTC suggested, however, that since the population might have declined prior to the beginning of the period, they could not conclude from the indices alone that the population is above MNPL.
A second analysis carried out by the IATTC was the Linked-Equilibria Method (LEM), an equilibrium reduction model similar to the one used for the back- calculation procedure (see Deriso and Anganuzzi, 1991). However, the LEM differs from traditional back-calculation models in the following ways:
(1) It does not attempt to model the transition between periods of stability in the population size;
(2) It does not require estimates of incidental mortality in the early years of the fishery;
(3) It does not require current estimates of absolute abundance; and
(4) It does not require estimates of the intrinsic rate of growth. Instead, the model incorporates the assumption that the population has been at equilibrium with the average incidental mortality during two different periods of time and an estimate of the relative change in population size between these periods.
An estimate of the intrinsic production of the spotted dolphin stock can be made if the stock is at equilibrium at two disjointed periods of time and an estimate is available of the fractional change in abundance between the two periods. The result is referred to as the LEM because it is a consequence of linking two equilibrium conditions with an estimate of fractional change in abundance.
The IATTC applied the method by assuming that the northeastern spotted dolphin stock was at equilibrium during the 1973-1977 period, which contains the first three years of the relative abundance indices used, and the 1986-1990 period. Using this technique, the median estimate of the relative abundance estimates for 1986-1990 divided by similar estimates for the 1973-1977 period is 61 percent (95 percent confidence limits of 55 and 68 percent), which is above, but close to, the MNPL threshold.
Response: NMFS believes that the IATTC was very conservative in rejecting the mortality data from 1959-1973 in their analyses.
Rejecting the mortality data from the time period when incidental mortality was likely to have the largest impact on the population is a biased approach. At a minimum, information on the number of sets from this time period should be used in combination with estimates of mortality from the early 1970s. Kill estimates derived in this manner should be negatively biased. Therefore if the back-calculation analysis or a similar type of modeling exercise indicated that the population is currently depleted, the interpretation should be that the population is depleted to a level no less than that predicted from the above analysis.
The participants at a recent Status of Stocks workshop where the LEM was reviewed (at DeMaster and Sisson), 1992), were concerned with the sensitivity of the LEM to parameters that were unknown, resulting in an inability to estimate current population status. Also, NMFS notes that the assumption of equilibria during the 2 time-periods is contradicted by available data. Furthermore, the reluctance by the IATTC to use mortality data from 1959-1973 in back-calculating abundance is inconsistent with their acceptance of assumptions concerning historical trends in abundance and mortality patterns used in the LEM analysis. The LEM should undergo the same rigorous evaluation that back-calculation has, to this date, it has not.
Status Determination
Northeastern Offshore Spotted Dolphin Incidental Mortality
Offshore spotted dolphins (S. attenuata) have been killed in the ETP tuna purse-seine fishery since, at least, 1959 (Perrin, 1969). Dolphin mortality increased in 1959, when the fleet began encircling dolphins as a fishing technique on a large scale (Joseph and Greenough, 1979). An estimated 391 sets were made on dolphins in that year (Punsley, 1983). It has been estimated that prior to the enactment of the MMPA in 1972, nearly five million dolphins were killed in the fishery, 3.4 million of which were offshore spotted dolphins (Lo and Smith, 1986; Wade, 1993a).
The IATTC provided NMFS with revised fisheries kill estimates for the northeastern spotted dolphin stock for the years 1973 to 1992, using the methods of Hall and Lennert (in press). The IATTC also provided NMFS with annual estimates of the number of sets on dolphins for 1959-1972, geographically stratified according to the reconfigured offshore spotted dolphin stock boundaries. This allowed Wade (1993b) to assess the population status of the northeastern spotted dolphin stock by calculating revised kill estimates for the years 1959-72, using the same methods as Lo and Smith (1986).
The majority of the estimated kill during the years 1959-72 was from the northeastern spotted dolphin stock. The location of dolphin sets during 1959- 1972 indicated that nearly all of the large kill of offshore spotted dolphin during that time period was from the northeastern spotted dolphin stock, as the fishery did not start to move offshore substantially until 1969 (Punsley, 1983). An estimated 3.0 million northeastern spotted dolphin were killed, representing an average of more than 200,000 per year for 14 years (Table 1). Following the enactment of the MMPA in 1972, the average annual mortality of the northeastern stock from 1971-1975 decreased to approximately 46,000 per year (Table 1).
The annual mortality declined further in 1976 because of a quota placed on the number of dolphins killed from all species by the U.S. fleet. In 1977, individual quotas for each stock were first imposed, which led to a dramatic decrease in the kill, which averaged about 6,000 northeastern spotted per year from 1977-84 (Table 1). In the mid-1980's, the tuna purse seine fleet became increasingly composed of non-U.S. boats, which were not subject to the quotas, but were required to maintain MPS rates that were similar to the U.S. fleet. This allowed the kill of dolphins to again increase, reaching a high of 52,000 northeastern offshore spotted dolphins in 1986, averaging about 32,000 per year from 1985-90 (Table 1).
Table 1.--Estimates of Fisheries Kill in Thousands by Year for the Northeastern
Stock of Offshore Spotted Dolphin (Stenella Attenuata). CV is the Coefficient
of Variation of the Kill Estimate. (Sources Specified at Wade, 1993a)
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Year Kill CV
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1959 ............................................. 15.9 .................. 0.53
1960 ............................................ 344.0 ................... .52
1961 ............................................ 366.0 ................... .48
1962 ............................................ 141.0 ................... .42
1963 ............................................ 158.2 ................... .36
1964 ............................................ 272.3 ................... .28
1965 ............................................ 318.5 ................... .29
1966 ............................................ 244.1 ................... .22
1967 ............................................ 171.8 ................... .23
1968 ............................................ 161.2 ................... .22
1969 ............................................ 271.5 ................... .22
1970 ............................................ 218.7 ................... .22
1971 ............................................ 111.3 ................... .22
1972 ............................................ 168.1 ................... .17
1973 ............................................. 49.9 ................... .18
1974 ............................................. 37.4 ................... .11
1975 ............................................. 49.4 ................... .18
1976 ............................................. 20.4 ................... .23
1977 .............................................. 5.9 ................... .12
1978 .............................................. 4.2 ................... .20
1979 .............................................. 4.8 ................... .17
1980 .............................................. 6.5 ................... .15
1981 .............................................. 8.1 ................... .19
1982 .............................................. 9.3 ................... .17
1983 .............................................. 2.4 ................... .27
1984 .............................................. 7.8 ................... .19
1985 ............................................. 26.0 ................... .12
1986 ............................................. 52.0 ................... .16
1987 ............................................. 35.4 ................... .12
1988 ............................................. 26.6 ................... .10
1989 ............................................. 28.9 ................... .11
1990 ............................................. 22.6 ................... .11
1991 .............................................. 9.0 ................... .11
1992 .............................................. 4.6 ................... .07
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Abundance of Northeastern Offshore Spotted Dolphin
NMFS completed five annual research vessel surveys between 1986 and 1990 (referred to as the Monitoring of Porpoise Stocks (MOPS) surveys) designed to estimate the abundance of cetaceans, particularly the abundance of spotted dolphin stocks (Wade and Gerrodette, 1992a). The data from the 1985-90 NMFS surveys were pooled to give a single best estimate of abundance for this newly defined northeastern spotted dolphin, which resulted in an estimate of abundance in 1988 of 730,900 animals (Table 2, Wade and Gerrodette, 1992b).
The population size of this new stock was much smaller than the abundance estimate made from the same data for the previously defined northern offshore spotted dolphin stock (Wade and Gerrodette, 1992a; 57 FR 27207, June 18, 1992). The estimates of northern offshore spotted dolphin from the MOPS surveys ranged from 658,300 to 2,205,500 per year of survey, with coefficients of variance (CVs) between 29 and 36 percent. The best, average estimate over the 5 years of MOPS surveys was 1,651,500 (CV = 21 percent) (57 FR 27207, June 18, 1992).
Given that the 1988 abundance estimate for the northeastern spotted dolphin was less than one-half of the estimate used in the proposed rule to designate the northern offshore spotted dolphin stock as depleted, and that the location of dolphin sets during 1959-1972 indicated that nearly all of the large kill of offshore spotted dolphin during that time period was from the northeastern stock, it became apparent that the northeastern spotted dolphin stock was likely to be at a lower relative population size than that estimated for the northern stock (Wade, 1993b).
Status of Northeastern Spotted Dolphins Relative to MNPL
One method for determining a population's status relative to MNPL is to estimate its historical abundance (Nh), meaning its abundance prior to significant fisheries mortality, which is assumed to be equivalent to the equilibrium population size (i.e., carrying capacity). The current population size is then compared with what is thought to be the MNPL for the population (Gerrodette and DeMaster, 1990). An estimate of historical population size for any spotted dolphin, Stenella spp., stock can be sensitive to a current abundance estimate, even one that occurs several decades later, because of their relatively low rate of increase (Smith and Polacheck, 1979). For a population that has experienced a relatively recent decline from known losses, such as the northeastern spotted dolphin, the estimate of Nh should be sensitive to the estimate of Nc (Gerrodette and DeMaster, 1990).
The entire time series of fisheries kill estimates through 1987 (Table 1) and the current abundance estimate for 1988 (Table 2) provided the necessary data to estimate historical population size in 1959 using the method of Smith (1983). To determine the status, Wade (in press) used these data, and the same methods and the same ranges for the parameters Rm and MNPL as Smith (1983), to estimate the historical population size for the northeastern spotted dolphin. The estimated relative population size was then used to assess the status of this stock.
Table 2.--Estimate of Abundance (in Thousands of Animals) of the Northeastern
Spotted Dolphin (Stenella Attenuata) from the Monitoring of Porpoise Stocks
(MOPS) Research Vessel Surveys, 1986-90 (Wade and Gerrodette, 1992b)
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From Abundance
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Northeastern spotted dolphin schools .................................... 663.3
Prorated from unid. spotted dolphins ...................................... 5.5
Prorated from unid. dolphins ............................................. 62.1
Total estimate .......................................................... 730.9
Standard error .......................................................... 103.6
Coefficient of variation ................................................ 0.142
Upper 95 percent confidence limit ....................................... 970.4
Lower 95 percent confidence limit ....................................... 588.7
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Confidence limits for the estimates of relative population size (current estimate relative to historical) were calculated using Monte Carlo simulation methods as developed by Wade (in press) for the recent stock assessment of the eastern spinner dolphin (Wade, in press).
Population Model: The methods of Wade (in press) were duplicated in Wade (1993b), using the simple recursive relationship
where
N subt =population abundance in year t
K subt =fisheries kill in year t
R subt =net recruitment rate in year t.
Density-dependence is incorporated into the equation through the net recruitment rate, which is defined as
where
R subm =maximum net recruitment rate
z=shape parameter that sets the maximum net productivity level (MNPL)
N subh =historical population size (assumed to be the equilibrium population size)
For any value of R subm and MNPL, z can be calculated as in Polacheck (1982). Equation 1 can be solved for N subt as a function of N subt+1 , R subt , and K subt . Therefore, by specifying an initial population size, the number of animals killed in each year, the maximum net recruitment rate, and the maximum net productivity level, these two equations can be iteratively solved for N subh . Because the abundance estimate was calculated from data pooled over 1986-90, the population trajectory was back-calculated from the mid-point of that time period, 1988, using the fisheries kill data through 1987.
Confidence Limits for Nh: For every combination of the parameters R subm and MNPL, confidence limits for relative population size were calculated following Buckland (1984). This was the same method used by Wade (in press) to estimate the precision of estimates of relative population size for the eastern spinner dolphin. For further details see Wade (in press).
Wade (1993a) estimated the kill in each year by multiplying stratified mortality-per-set rates from pooled 1964-1972 observer data by the numbers of sets on dolphin in each stratum in each year. Therefore, the kill estimates for 1959-1972 were not independent from each other. For each simulation iteration the kill values for 1959-1972 were randomly generated using the same random deviate. This resulted in the kill values for those years being perfectly correlated amongst themselves from simulation trial to trial, which correctly reflected the lack of independence in the actual estimates. The kill values for all other years were sampled independently.
Estimates of R subm and MNPL: R subm can be estimated from appropriate life history data. The northern offshore spotted dolphin was estimated to have an age of sexual maturity (ASM) of 12.2 years and a calving interval (CI) of 3 years (Myrick et al., 1986). Fortunately, nearly all the dolphins used in that study were from the northeastern spotted dolphin stock area, and a re- calculation of Myrick et al.'s (1986) data excluding the few animals that were outside the northeastern stock area did not change the estimate of ASM (at Wade, 1993b). Therefore, the estimates of Myrick et al. (1986) can be used as estimates for the northeastern spotted dolphin stock.
An ASM of 12 years and a CI of 3 years leads to an estimate of the rate of increase of about 0.04 from Reilly and Barlow (1986), using the highest adult survival rate they considered of 0.97. Barlow and Boveng (1991) used the same estimates for ASM and CI with more realistic survival curves based on model life tables from other species, to give estimates for R subm between 0.972 and 0.042. If the true values for the population were as low as an ASM of 10 years and a CI of 2 years, the estimate of R subm would be about 0.06, again using the highest considered survival rate of 0.97 from Reilly and Barlow (1986). Therefore, 0.06 was likely the highest possible value of R subm for this population, and the rounded value of 0.04 from Barlow and Boveng (1991) can serve as a point estimate. Wade (1993b) used values ranging from 0.01 to 0.06 by increments of 0.01, for a total of 5 values. The latter value (0.06) was the same maximum value used by Smith (1983) and Wade (in press).
No direct estimate of MNPL exists for the northeastern spotted dolphin. Values used by Smith (1983) for MNPL were 0.50, 0.65, and 0.80 (MNPL is expressed as a fraction of equilibrium population size in this paper), corresponding to z values (see Eq. 4) of 1.0, 3.482, and 11.216, respectively. These encompassed the range of estimated values of MNPL for long-lived marine mammals, such as dolphins, based on work by Fowler (1981). Fowler (1984) gave evidence the MNPL was greater than 0.50 for cetaceans. A value of 0.60 is currently being used for management of cetaceans under the MMPA (45 FR 64548, Oct. 31, 1980). Wade (1993b) considered this value the best point estimate of MNPL currently available for the northeastern spotted dolphin. Values of z were used so that MNPL ranged from 0.50 to 0.80 (the same range as in Smith, 1983), using increments of 0.05, for a total of 7 values. The 6 values used for R subm and the 7 values used for MNPL produced a total of 42 parameter combinations for which relative population size was estimated.
Relative Population Size from the Model: Relative population size (N subc /N subh ) ranged from 0.19 to 0.28 (Table 3, at Wade, 1993b). Relative population size increased with both R subm (recruitment rate) (Fig. 2) and MNPL (the amount of non-linearity in the density-dependence response). The lowest relative population size of 0.19 was for the case of a value of 0.01 for R subm (i.e., 1 percent net growth in the population before fisheries kill was included) and a value of 0.50 for MNPL. The highest relative population size of 0.28 was for the case of the highest Rm of 0.06 and a value for MNPL of 0.80. These low and high estimates of relative population size corresponded to estimates of pre-exploitation abundance in 1959 of 3,827,000 and 2,573,000 respectively. There were no combinations of parameter values such that relative population size was estimated to be above MNPL. Using point estimates of 0.04 for R subm and 0.60 for MNPL resulted in an estimated relative population size of 0.23, with an estimate of N subh of 3,141,000.
The upper 95 percent confidence limit (CL) for relative population size as a function of R subm and MNPL, based on the sampling error of the abundance and kill estimates, ranged from 0.36 to 0.61 (Table 3, at Wade, 1993b). The upper confidence limit was also below the value used for MNPL for all parameter combinations. The lower 95 percent CL for relative population size as a function of R subm and MNPL, ranged from 0.12 to 0.17.
When the point estimates of 0.04 for R subm and 0.60 for MNPL were used, the population trajectory declined until 1977 (Fig. 3), at which time the estimated fisheries kill declined substantially (Table 1). The population trajectory showed a slightly increasing trend from 1978 to 1986, and then declined slightly again in 1987 and 1988 (Fig. 3).
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Relative Abundance Estimates From Tuna Vessel Observer Data
In addition to MOPS survey estimates of absolute abundance, an independent assessment of the population trend of the northeastern spotted dolphin is available from the population abundance index calculated by Anganuzzi and Buckland (1993) from tuna vessel observer data. Sighting data collected by observers on the tuna vessels are currently considered the most reliable for monitoring trends in the abundance of spotted dolphins (Anganuzzi and Buckland, 1989; DeMaster et al., 1992).
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The estimated population trends in Anganuzzi and Buckland (1993) starts in 1975, and shows a statistically significant decline until 1983. This represents a population decline of 35 percent over those eight years (data also presented at 57 FR 27207, June 18, 1992). This population trajectory declines for several more years than the population trajectory calculated here, but this may be explained by the fact that the population model used here does not take into account the age structure of the kill, as discussed in Wade (in press) and Goodman (1984). The fisheries kill was biased towards mature animals (Barlow and Hohn, 1984), meaning that after the fisheries kill dropped in 1977 to a level that would allow for positive population growth at stable age distribution, the population apparently continued to decline because of the natural lag induced by the relatively old ASM of 12 years. The population may only have been able to start growing when a sufficient number of females had become sexually mature, apparently around 1984 according to the population trend of Anganuzzi and Buckland (1993).
Although the estimates of Anganuzzi and Buckland (1993) indicated the population grew between 1983 and 1986, it has apparently been fairly level since 1986, at a level in 1991 still significantly below the population level in 1976, with the population estimated to be at approximately 75 percent of its size in 1976. Given the large fisheries kill known to have occurred before 1975 (Table 1), this would be sufficient evidence in itself to indicate that the northeastern stock of offshore spotted dolphin was depleted. A population that, as of 1975, had sustained an average fisheries kill of an estimated 190,000 animals per year for 16 years (Table 1), and then declined another 25 percent to a level of about 730,000 animals, is highly likely to be at less than 60 percent of its initial population size in 1959.
The lack of significant growth between 1988 and 1992 (Anganuzzi and Buckland, 1993) indicates that the status as calculated for 1988 is substantially the same in 1992 (Wade, 1993b). Fisheries kill estimates in 1988, 1989, and 1990 (26,625, 28,898, and 22,616, respectively) were between 3- 4 percent of the population abundance estimate of 730,900, which would have prevented any substantial population growth given the estimated rate of increase of only 4 percent. The fisheries kill declined dramatically in 1991 and 1992 to only 9,005 and 4,636, respectively, values of approximately 1 percent and 0.5 percent of the population size. This evidence indicates that the population should have increased slightly between 1990 and 1992, which was supported by relative trend data of Anganuzzi and Buckland (1993). However, the population could only have grown by about 6 percent in total, which would not make the status in 1992 significantly different from the status in 1988. If fisheries kill remains at the current lower level of less than 5,000 per year, the population should eventually increase.
Although there are some uncertainties (e.g. with the early mortality data), the results of Wade (1993b) indicate that the northeastern spotted dolphin population was well below historical abundance levels in 1988, such that it can be concluded that, as of 1988, the northeastern stock of offshore spotted dolphins was depleted as defined by the MMPA. The substantial fisheries kill that occurred between 1988 and 1990 makes it highly unlikely that the population has experienced any significant recovery since then. The trends shown by Anganuzzi and Buckland (1993) thus corroborate the analysis by Wade (1993b), giving a second, independent assessment demonstrating that the population is depleted.
Final Determination Under the MMPA
A determination of depletion must, in significant part, be based on the relationship between the optimum carrying capacity (K) and OSP, as described in the MMPA. MNPL is considered the lower end of OSP, and NMFS has adopted by regulation a value for MNPL that is at 60 percent of K (42 FR 64548, Dec. 27, 1977 and 45 FR 72178, Oct. 31, 1980).
The population size in 1988 of northeastern spotted dolphins was estimated to be well below MNPL for all parameter values used in this study by Wade (1993b). Because the ranges of values used should span the true values for this stock, the uncertainty in these values does not prevent a definitive conclusion from being reached. At best, the population was estimated to be at only 28 percent of its 1959 population size. As can be seen from the population trajectory (Fig. 3), this decline was due mostly to the large fisheries kill that occurred between 1960 and 1976. For all 42 parameter combinations, the population was estimated to be at less than half of MNPL, indicating the stock is not close to being at OSP. For the point estimates of 0.04 for Rm and 0.60 for MNPL, the population was estimated to be at 23 percent of historical population size, far below the MNPL value of 60 percent.
Wade (1993b) showed that in the calculation of confidence limits for relative population, the precision of the estimates was sufficient to make a status determination. Incorporating the sampling error of the current abundance estimate and the fisheries kill estimates, Wade (1993b) provided confidence limits around the estimated relative population sizes (Fig. 2). In all cases the upper 95 percent confidence limit was still below MNPL.
The confidence limits around relative population size were all greater proportionally than the confidence limits around N subc . From the simulation, it was also possible to calculate a coefficient of variation (CV) for relative population size, in addition to the confidence limits. These estimated CV's for relative population size ranged from 29 percent to 37 percent much larger than the 14 percent CV on the current abundance estimate. The CV of relative population size reflects the large uncertainty in the fisheries kill estimates, particularly in the early years. However, if it can be assumed that there was no large positive bias in the fisheries kill estimates, then the imprecision of the fisheries kill estimates was not so large as to prevent a definitive stock assessment. Any potential biases in the fisheries kill appear to be negative, indicating that fisheries kill may have been under-estimated (Wade, in press; Wade, 1993a). An underestimate of fisheries kill would lead to an overestimate of relative population size, which would not change the result from a management perspective, as the population would still be considered depleted.
The accuracy of the fisheries kill estimates prior to 1971 depended on several important assumptions, the primary one being that the stratified MPS rates were constant from 1959-72 (Wade 1993a). However, it must be emphasized that the number of sets made on dolphin for that time period were known with very high precision (Punsley, 1983), and what was lacking were more observations of MPS prior to 1971. The kill estimates were made based on only 3 observed trips prior to 1971 pooled with 17 observed trips from 1971-72, although there is information from at least four other trips prior to 1971 that their kill rates were consistent with the observed trips (Wade, 1993a). Because the three observed trips prior to 1971 were not part of an established observer program (Smith and Lo, 1983), Wade (1993) showed that removing the data from those three trips had an insignificant effect on the kill estimates. However, it will probably never be possible to conclusively test all the assumptions implicit in estimating kill for that time period. Therefore, it is worthwhile to ask the question, how biased must the kill estimates for that time period have been for the stock to not currently be depleted? To answer that question, Wade (1993b) solved for how much lower the 1959-70 mortality would have to have been for the population to currently be at MNPL by re- calculating relative population size assuming Rm of 0.04 and an MNPL of 0.60. He found that for the northeastern stock to be at MNPL, 1959-70 mortality would have to be 12 percent of the estimates in Table 1. This means that the kill would have to have been overestimated by almost an order of magnitude, extremely unlikely given the fairly similar MPS rates observed in 1973, for which much more data were available.
Wade (1993b) addressed this issue in another way by re-calculating estimates of the 1959-72 kill using the methods of Wade (1993a) but using the greater amount of data available by pooling 1971-73 observer data, rather than using the pooled 1964-72 observer data. These were likely underestimates of the kill because of the decline in MPS in 1973. Using these kill estimates and the same values as above for Rm and MNPL, the northeastern stock of offshore spotted was estimated to be at a relative population size of 0.46. Wade (1993b) then estimated 1959-72 kill by multiplying the number of dolphin sets in the northeastern stock area (at Wade, 1993a) by the average dolphin MPS from 1974 (the first year a formal randomized design was used to place observers on fishing vessels) as calculated from Wahlen (1986), prorating by the observed 1991-72 proportion of spotted dolphins in the kill of 0.964. Using these estimates of the kill, the northeastern spotted dolphin stock was still estimated to be at a population size of less than MNPL. The population was thus also estimated to be depleted even using these later kill rates, which were undoubtedly lower than the kill rates before the Enactment of the MMPA in 1972. It can be concluded the northeastern stock of offshore spotted dolphin was not estimated to be depleted because of an artifact of the small sample size of observer data available prior to 1973.
Again, Wade (1993b) determined that the relative population size (N subc /N subh ) ranged from 0.19 to 0.28. The lowest relative population size of 0.19 was for the case of a value of 0.01 for R subm (i.e., 1 percent net growth in the population before fisheries kill was included) and a value of 0.50 for MNPL. The highest relative population size of 0.28 was for the case of the highest R subm of 0.06 and a value for MNPL of 0.80. There were no combinations of parameter values such that relative population size was estimated to be above MNPL. Using point estimates of 0.04 for R subm and 0.60 for MNPL resulted in an estimated relative population size of 0.23.
Therefore, NMFS has determined that the northeastern stock of offshore spotted dolphin is below OSP, and by definition, is depleted under the MMPA.
Conservation Plan for Northeastern Offshore Spotted Dolphins
Section 115(b)(1) of the MMPA requires NMFS to prepare a conservation plan for any species or stock designated as depleted unless it determines that such a plan will not promote the conservation of the species or stock. When the eastern spinner dolphin was determined to be depleted (58 FR 45066, Aug. 26, 1993), NMFS explained its rationale for determining that a conservation plan would not promote the conservation of that stock, at that time. Similarly, NMFS does not plan to develop a conservation plan for the northeastern spotted dolphin stock at this time because it would not further promote the conservation of the species.
Existing regulatory mechanisms and international agreements protect ETP dolphins under the MMPA, and preclude the immediate need for a conservation plan. International and U.S. efforts to reduce dolphin mortality in the purse- seine fishery for tuna, and to promote dolphin conservation, have been, or are being, implemented. In a series of intergovernmental meetings convened under the auspices of the IATTC in 1991 and 1992, nations harvesting tuna in the ETP have agreed to limit dolphin mortality to levels approaching zero. The nations have committed to:
(1) Achieving 100-percent observer coverage;
(2) Identifying alternative fishing methods that would not involve the encirclement of dolphins and, therefore, would not result in dolphin mortality associated with purse-seine techniques;
(3) Reducing dolphin mortality; and
(4) Developing and implementing a dolphin conservation program in 1992 and subsequent years (57 FR 21081, May 18, 1992.
Also, the International Dolphin Conservation Act (Pub. L. 102-523) was enacted on October 26, 1992. This act amended the MMPA to authorize the United States to enter into an international agreement to establish a global moratorium to prohibit harvesting of tuna through the use of purse-seines deployed on or to encircle dolphins for at least 5 years beginning on March 1, 1994.
These protective measures are considered adequate to protect the species from further declines within the foreseeable future.
Statues of Western/Southern Offshore Spotted Dolphin
The northern offshore spotted dolphin stock consisted of animals from the current northeastern spotted dolphin stock and, to a lesser extent, the current western/southern stock of offshore spotted dolphin. This listing considered only the current northeastern stock of spotted dolphin. However, NMFS is continuing to examine data collected within the range of the western/southern stock of spotted dolphin to determine whether it should also be considered for listing under the MMPA.
Classification
The Assistant Administrator for Fisheries, NOAA, has determined that this final rule is exempt from the requirements of Executive Orders 12612 and 12866, the Paperwork Reduction Act, and the Regulatory Flexibility Act, because section 115(a)(2) of the MMPA requires listing decisions to be based solely on the basis of the best scientific information available.
A designation of depletion in this instance, which is similar to a listing action under ESA section 4(a), is categorically excluded by NOAA Administrative Order 216-6 from the requirement to prepare an environmental assessment or an environmental impact statement under the National Environmental Policy Act.
References
References are provided upon request, see ADDRESSES, or can be found at the following:
Wade, P. 1993a. Revised estimates of fisheries kill of dolphin stocks in the eastern Tropical Pacific, 1959-1972. NOAA Admin. Rept. LJ-93-17, NMFS Southwest Fish. Sci. Center, La Jolla, CA, 92038. 20 pp.; and
Wade, P. 1993b. Assessment of the northeastern stock of offshore spotted dolphin (Stenella) (Attenuata). NOAA Admin. Rept. LJ-93-18, NMFS Southwest Fish. Sci. Center, La Jolla, CA, 92038. 21 pp.
List of Subjects in 50 CFR Part 216
Administrative practice and procedure, Imports, Indians, Marine mammals, Penalties, Reporting and recordkeeping requirements, Transportation.
Dated: October 26, 1993.
Nancy Foster,
Deputy Assistant Administrator for Fisheries.
For the reasons set out in the preamble, 50 CFR part 216 is amended as follows:
PART 216--REGULATIONS GOVERNING THE TAKING AND IMPORTING OF MARINE MAMMALS
1. The authority citation for part 216 continues to read as follows:
Authority: 16 U.S.C. 1361 et seq., unless otherwise noted.
2. In §216.15, a new paragraph (f) is added to read as follows:
§216.15 Depleted species.
* * * * *
(f) Northeastern Offshore Spotted Dolphin (Stenella attenuata).
(FR Doc. 93-26721 Filed 10-29-93; 8:45 am)
BILLING CODE 3510-22-M