Chapter IV — Marine Mammal-Fisheries Interactions 
eastern spinner dolphin as depleted. The Service 
based the proposed determination in large part on data 
collected from population surveys it had conducted in 
the eastern tropical Pacific between 1986 and 1990. 
When the best estimate of the current population size 
from those surveys, 565,800, is compared to an 
estimate of the pre-exploitation population size made 
in 1979, the population is considered to be at 33 
percent of its carrying capacity, well below its maxi- 
mum net productivity level. 
The Service published a proposed rule on 18 June 
1992 to designate the northern offshore spotted 
dolphin as depleted. The best estimate of the current 
size of this dolphin population derived from the 1986- 
1990 survey cruises, 1,651,600, was determined by 
the Service to be 27 percent of the 1979 estimate, or 
about 23 percent of its carrying capacity. 
The Marine Mammal Commission, in consultation 
with its Committee of Scientific Advisors, commented 
on the proposed depletion designations by letter of 28 
August 1992. The Commission noted that the best 
available estimates of eastern spinner and northern 
offshore spotted dolphin abundance are those derived 
from the multi-year Monitoring of Porpoise Stocks 
(MOPS) surveys as revised following the November 
1991 workshop on the status of eastern tropical 
Pacific dolphin stocks. When the 1991 estimate of 
eastern spinner abundance is compared with the best 
available estimate of its pre-exploitation stock size, it 
is clear that the eastern spinner dolphin is well below 
its probable maximum net productivity level (MNPL) 
and should be designated as depleted. 
While no pre-exploitation abundance estimate for 
the northern offshore spotted dolphin was provided in 
the proposed rule, the Commission found that the 
Service had made a credible case that the stock is 
depleted. The proposed rule indicated that significant 
numbers of northern offshore spotted dolphins were 
killed incidental to the yellowfin tuna fishery during 
the 1960s and early 1970s. The high level of inciden- 
tal mortality during that period almost certainly caused 
a marked reduction in the abundance of the stock 
prior to 1975. Relative abundance estimates presented 
in the proposed rule indicate that the stock has de- 
clined considerably further since the mid-1970s and 
is, in all likelihood, below its maximum net productiv- 
107 
ity level. Nevertheless, the Commission recommend- 
ed that the Service in its final rule discuss the likely 
magnitude of the stock’s decline during the 1960s and 
1970s and provide the Service’s best estimate of the 
stock’s pre-exploitation abundance. 
In further support of the proposed depletion 
findings, the Commission noted that there is no reason 
to believe that the carrying capacity of the eastern 
tropical Pacific Ocean ecosystem has been reduced 
during the past three decades. Thus, it does not 
appear that the observed declines in these dolphin 
stocks are a result of environmental change. 
The Commission also recommended that the final 
tule discuss the possible consequences of the depletion 
designations and describe any actions the Service 
would take following issuance of the findings. In this 
regard, the Commission noted that section 115(b)(1) 
of the Marine Mammal Protection Act requires the 
Service 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. The Commission asked that the 
Service indicate in the final rule whether it intends to 
prepare conservation plans for the eastern spinner and 
northern offshore spotted dolphins. If the Service 
decides not to prepare such plans, the Commission 
requested that the Service explain its rationale for 
determining that the plans would not promote the 
conservation of the stocks and describe what actions 
it intends to take (e.g., continued monitoring) in 
response to the depletion findings. 
The Commission also noted that many of the 
analyses presented in the proposed rules, including 
analysis of data from the dolphin monitoring surveys, 
have only recently become available. The Commis- 
sion therefore requested that the Service promptly 
review the available information for other dolphin 
stocks, particularly the northern stock of the common 
dolphin and the coastal stock of the spotted dolphin, 
and advise the Commission as to whether other 
depletion designations may be warranted. 
On 2 September 1992 the Service published a 
Federal Register notice presenting new information on 
the structure of spotted dolphin stocks that occur in 
the eastern tropical Pacific Ocean. Based upon two 
