Whale depredation
Depredation of catch from longline fishing gear by marine mammals, particularly by toothed whales (suborder Odontoceti), is a growing challenge both to commercial fishers as well as researchers surveying fish stock abundance (Sigler et al. 2008). It is suspected that this behavior is increasing, partially due to the cessation of large-scale whaling, and also due to the introduction of quota-based fishing (SEASWAP, unpublished data)1 that has made captured fish available to whales over a much longer fishing season each year, compared to the short derby fisheries. Mammals targeting longline gear can reduce catch rates, damage gear and catch and ultimately cost fishers time, fuel, and money. Over the long term, depredation could lead to the loss of productive fishing grounds.
The Pacific halibut fishery has experienced various challenges with respect to depredators over the years. In the 1950s and 1960s, sea lion (suborder Pinnipedia) depredation of Pacific halibut was a problem for the fishery. For many years, seal bombs were a common item taken aboard fishing vessels to deter sea lion depredation during gear haul back. Over the past decade, the IPHC has been receiving more and more reports of whale depredation events occurring on the fishing grounds, and some harvesters have reported being driven off traditionally productive grounds as a result. The IPHC fishery-independent setline survey (FISS) has also encountered depredation events over the years.
Categorizing whale interactions can help researchers understand potential impacts of this behavior on their data sets and provide meaningful observations for the fleet in general. Since 2009 the IPHC has been collecting more refined data regarding whale interactions on its FISS (Dykstra and Soderlund, 2010). Preliminary analysis suggest that killer whale (Orcinus orca) interactions are associated with decreases in target catch rates, as well as substantial damage to both gear and captured fish. Sperm whale (Physeter macrocephalus) interactions are more difficult to quantify, with less obvious damage to the catch (Wong, 2015). However, the data indicate that while there is less-obvious damage to the catch compared to killer whales, increased gear damage is incurred when sperm whales are present during hauling operations. These data can be used to determine when survey data should be excluded from stock assessment analyses due to depredator interactions, and provides valuable observations for other agencies that are also struggling to resolve depredation impacts within their survey data, as well as for individuals who are looking to identify ways to minimize depredation events. IPHC continues to work on avenues to better categorize and quantify the effect of depredation, including log book modifications and other ongoing research.
References
Dykstra, C.L., and Soderlund, E.D. 2010. Categorizing marine mammal depredation on IPHC standardized setline surveys. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2010:435-444.
Sigler, M., Lunsford, C. 2008. Sperm whale depredation of sablefish longline gear in the northeast Pacific Ocean. Mar. Mammal Sci. 24: 16-27.
Wong, N. 2015. Marine mammal depredation on IPHC standardized setline surveys: a look at killer whales and sperm whales as major depredators in Alaskan waters. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2015:418-441.
1Sperm whale and longline interactions. http://seaswap.info/background/ Retrieved September 2017.