MARINE MAMMAL COMMISSION - Annual Report for 1995 



therefore have been done on shore when animals haul 

 out, roost, or strand. Interaction rates from land- 

 based studies, however, do not account for animals 

 killed at sea, and probably reflect only those that 

 interact with debris close to shore or that sustain mild 

 impacts (e.g., entanglement in small pieces of debris). 

 Nevertheless, analyses to date suggest that some 

 populations are impacted significantly. 



Some seal species, such as the northern fur seal 

 and the Hawaiian monk seal, appear to be the marine 

 mammals most affected by marine debris. Studies of 

 the world's largest northern fur seal population, the 

 fur seal herd on the Pribilof Islands, suggest that late 

 in the 1970s up to 50,000 juvenile fur seals per year 

 may have been entangled and killed annually by 

 marine debris, and that entanglement was a principal 

 cause in a six to eight percent annual decline in that 

 population in the 1970s and early 1980s. More 

 plausible explanations for the decline have not been 

 postulated and, given the population's failure to 

 recover over the past decade, it is possible that 

 entanglement is still a problem for this population. 



Observations from seasonal field camps established 

 to study Hawaiian monk seals, one of the world's 

 most endangered seals, also suggest potentially serious 

 entanglement problems. Observed entanglement rates 

 at the species' major colonies typically are less than 

 one percent of a colony per year, but rates of up to 

 7.5 percent per year have been recorded, and field 

 camps usually are only in place for a few days to a 

 few months. Entangled seals are routinely disentan- 

 gled when found and, while entangling material is 

 often loose, suggesting seals might have been able to 

 free themselves, in some cases it is firmly attached 

 and would likely have remained attached without 

 intervention. Considering the short period of observa- 

 tion and the probability that some seals are entangled 

 and killed at sea unobserved, these incidents could 

 reflect a significant problem for this species. 



Two other U.S. marine mammal populations for 

 which marine debris may be a particular concern are 

 Florida manatees and western North Atlantic northern 

 right whales. Analyses of photo catalogues for both 

 populations suggest that 3.6 percent of the manatee 

 population and 57 percent of the northern right whale 

 population bear scars from entanglement incidents. 



Carcass salvage programs for these populations also 

 report that about 1.7 percent of manatee carcasses and 

 8 percent of the right whale carcasses are the result of 

 entanglements. In both cases, most incidents probably 

 involve interactions with active rather then derelict 

 fishing gear, but distinguishing between these sources 

 is not possible. Some manatee deaths due to ingestion 

 of marine debris also have been documented. 



Other species that may have high levels of impact 

 from marine debris include sea turtles and certain 

 species of commercial shellfish. Studies to tag 

 loggerhead turtles in the eastern North Atlantic Ocean 

 have found six percent of the turtles captured at sea 

 entangled in debris. Considering the effects of such 

 a burden on turtle metabolic requirements and their 

 increased vulnerability to predators, as well as the 

 species' penchant for ingesting plastic, it seems 

 probable that marine debris is a significant conserva- 

 tion issue for this population and sea turtles generally. 

 As discussed below, the accumulation of derelict crab 

 and lobster pots and gillnets also may pose significant 

 entrapment potential for commercially valuable 

 shellfish stocks. 



In light of marine debris impacts on marine 

 mammals and their ecosystems, the Marine Mammal 

 Commission has continued to assist other agencies and 

 groups in addressing the problem. Major efforts in 

 this regard in 1995 are discussed below. 



Derelict Fishing Gear 



During the course of commercial fishing opera- 

 tions, derelict fishing gear may be generated by at 

 least eight factors — the weather (e.g., storms and ice 

 conditions), bottom snags, ship collisions, fishing 

 methods, human error, vandalism, gear failure, and 

 deliberate discards. Many of the types of marine 

 debris most hazardous for marine mammals and other 

 species (e.g., netting, rope, and monofilament line) 

 are produced by commercial fisheries. Although 

 entanglement of animals in small pieces of netting and 

 line is the major source of entanglements reported by 

 land-based observers, the catch of animals in relative- 

 ly intact fishing gear lost and discarded at sea may be 

 a greater source of mortality. 



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