ACS Research Committee Report

Sublethal Predator Effects
Source: Wirsing, A.J., Heithaus, M.R., Frid, A., and Dill, L.M. 2008. Seascapes of fear: evaluating sublethal predator effects experienced and generated by marine mammals. Mar. Mamm. Sci. 24(1): 1-15.

Predators affect prey populations through killing and consuming individuals, but also through indirect effects, such as intimidation.

Indirect predator effects have been well documented in the terrestrial environment, but poorly so in the marine ecosystem. Prey species must weigh the benefits accrued through use of a resource patch with the risk of being killed by a predator during foraging.

If predation pressure is negligible, then we would expect the prey species to forage in a way that maximizes energy gain. However, in situations where predator risk and resource abundance are positively correlated, the prey species must balance the risks and benefits of foraging in a certain location.

This paper described three marine mammal case studies, one cetacean, one pinniped, and one sirenian. Only the cetacean case study, bottlenose dolphins in Shark Bay, Australia, is detailed here.

Bottlenose dolphins in Shark Bay forage in two major habitats: deep channels and shallow banks. Shallow banks consistently have greater fish biomass, but also a higher abundance of the dolphins' only local predator, the tiger shark.

Over a 3-year period, dolphins were found to consistently allocate their time to shallow and deep foraging patches according to fish availability during two winters when tiger sharks were scarce. However, dolphins allocated more time to deep habitats during three summers and one winter when sharks were common.

From this, it can be concluded that deep waters are perceived by the dolphins as safer from shark predation. Dolphins sacrifice feeding efficiency in exchange for safety, and by causing dolphins to shift their distribution, tiger sharks create a temporary haven for fish in the shallow banks.

Dolphins also alter their distribution within shallow bank areas during periods of high shark abundance by choosing to forage on the periphery of sea grass beds, where escape into deeper water is easier. These areas are also most used by tiger sharks, indicating that dolphins choose areas of easier escape rather than areas of fewer possible encounters.

The case studies analyzed here suggest that marine mammals are subject to the non-consumptive effects of predators, even though direct observations of predator kills are uncommon. Predator effects can be important in identifying critical habitats for endangered marine mammals. In addition, human disturbance could be perceived as a predator effect; therefore these studies that determine the level of predator effects may also be used to determine the level of human disturbance. 

Sperm Whale Depredation of Longline Gear
Source: Sigler, M.F., Lunsford, C.R., Straley, J.M., and Liddle, J.B. 2008. Sperm whale depredation of sablefish longline gear in the northeast Pacific Ocean. Mar. Mamm. Sci. 24 (1): 16-27

Sperm whale depredation (removal of catch) from longlines has been documented in Alaska and southern oceans.

In Alaska, the primary prey is sablefish, which is caught using longline gear set on the bottom. Fishermen report reduced sablefish catch when sperm whales are present and they see characteristic damage, such as missing body parts, crushed tissue, and blunt tooth marks on the fish.

The goal of this study was to estimate the magnitude of sperm whale depredation in the Alaskan sablefish longline fishery and whether it has increased in recent years.

Data was collected in the Gulf of Alaska from 1998 to 2004 during annual sablefish longline surveys conducted by the National Marine Fisheries Service. Frequency of occurrence was calculated as the number of sampling days when sperm whale depredation was observed divided by the number of days when sperm whales were observed.

In addition, the percentage of damaged sablefish per longline set was calculated. Sablefish catch rates were compared between stations where depredation occurred and those where none occurred in order to determine the impact of sperm whale depredation on the fishery.

Sperm whales were observed on 16% of the longline survey days. Depredation was common when sperm whales were sighted; damaged sablefish were documented at 65% of locations with whale sightings. However, few (only 0.8% average) fish were damaged even in areas where depredation was present.

The number of sperm whales and the percentage of depredation documented varied significantly between years. Sablefish catch rates were less where depredation occurred, but the difference was not significant. Depredation accounted for less than 1% of annual longline fishery catch.

The prevalence of depredation varies by year, but does not appear to be increasing over time. Anecdotally, fishermen observed higher rates of depredation when fish were not headed and gutted at sea; sperm whales have been documented consuming this fishery offal and it may be a preferred prey source.. 

Acoustic Signals of Delphinids in the western North Atlantic and northern Gulf of Mexico
Source: Baron, S.C., Martinez, A., Garrison, L.P., and Keith, E.O. 2008. Differences in acoustic signals from Delphinids in the western North Atlantic and northern Gulf of Mexico. Mar. Mamm. Sci. 24(1): 42-56.

Variation in whistle patterns has been documented between different delphinid species; this variation may be related to physiological, ecological, or behavioral differences between animals.

This study examined whistles from pilot whales, Atlantic spotted dolphins, and bottlenose dolphins in U.S. waters to evaluate whistle differences within species between different geographic areas.

Recordings were collecting during marine mammal surveys conducted by the National Ocean and Atmospheric Administration between 2002 and 2004.

Whistles of spotted dolphins on the continental shelf were significantly different from those in offshore areas. They were different in frequency, band width, and call structure.

These populations are adjacent to one another and may geographically overlap in certain parts of their range. These call differences likely represent a social, rather than strictly geographic, isolation; similar call differences are seen in sympatric populations of killer whales near British Columbia, Canada.

Atlantic bottlenose dolphin whistles were significantly different in duration and call structure from Gulf of Mexico bottlenose. These populations are geographically completely isolated, meaning that vocal learning is influenced exclusively by members of the same population leading to divergent whistle characteristics.

There was a low sample size for pilot whale acoustic recordings, but overall there was no significant difference in whistles of pilot whales in the Atlantic versus the Gulf of Mexico. This result may be due to the low sample size and therefore a reduced statistical power of this analysis or it may indicate mixing of the two populations.

Further research on this species is necessary to determine the validity of these results. Intraspecific differences in whistle characteristics found in this study may be due to social isolation leading to divergent whistle development or it may be due to these species altering their sounds based on the oceanographic variables in their respective habitats. 

Strand Feeding in Bottlenose Dolphins
Source: Duffy-Echevarria, E.E., Connor, R.C., and St. Aubin, D.J. 2008. Observations on strand-feeding behavior by bottlenose dolphins (Tursiops truncatus) in Bull Creek, South Carolina. Mar. Mamm. Sci. 24(1): 202-206.

Bottlenose dolphins are highly adaptable and have been documented to use site-specific feeding behaviors. 

Interactions between Artisanal Fisheries and Bottlenose Dolphins
Source: Brotons, J.M., Grau, A.M., and Rendell, L. 2008. Estimating the impact of interactions between bottlenose dolphins and artisanal fisheries around the Balearic Islands. Mar. Mamm. Sci. 24(1): 112-127.

Interactions between marine mammals and fisheries are found in many different species, geographic areas, and fishery types; these interactions are a serious conservation concern.

In some cases, marine mammals have learned to exploit fishing gear to obtain prey and these behaviors can spread quickly throughout populations.

These interactions are a problem in the Mediterranean, where bottlenose dolphins interact with artisanal fisheries. Artisanal fisheries near the Balearic Islands in the Mediterranean are small-scale and operate from 5-9 m vessels using bottom-set nets to depths up to 200 m.

The soak time for these nets is 30 min to 3 days, depending on target species and weather conditions. These nets fish for spiny lobster, cuttlefish, and striped surmullet, but there is bycatch of other species as well.

The Balearics artisanal fishing fleet is made up of 296 vessels, employs 501 people, and lands 319 metric tons of fish annually. The interactions between artisanal fisheries and bottlenose dolphins have been increasing since 1990.

Most interactions involve dolphins depredating nets, which leads to reduced catch and possible gear damage for the fishermen and increased entanglement risk for the dolphins.

Between January 2001 and April 2003, a total of 1040 net sets were observed in this study. Dolphin depredation was recorded on 138 (13%) of sets; depredation was either seen directly or assumed based on evidence such as fish remains in the nets.

The probability of documenting depredation was much higher when dolphins were sighted during a haul (67%) than when dolphins were not sighted (7%).

Modeling of depredation indicated that area, year, month, net type, and time of day were all important factors. There was an increase in depredation off West Mallorca from 2001 to 2003 when it reached 75% of all sets; this increase was not found in other areas of the study. A possible reason for this difference is localized prey depletion or increased number of dolphins that specialize in net depredation in this area.

Depredation was more frequent during winter, especially December and January, and less frequent from July to September. This seasonal difference may be due to reduced prey abundance or a more inshore distribution of dolphins during the winter months.

Depredation also peaked between midnight and 2 a.m. and between 2 p.m. and 4 p.m.; it was at its lowest in the early morning. This diel variation likely relates to concurrent activity patterns of the dolphins.

Finally, nets with a mesh size of 100-200 mm were much less likely to suffer depredation than other size nets. This sized net is primarily used for lobsters, which are not a typical prey source for dolphins.

Models of the economic impact of depredation found an average loss of 2.7% of the catch value and 3.4% of the catch biomass for the observed subset of fishing effort. The damage to fishing nets due to depredation may have as much of an economic impact on fishermen as catch loss.

Two dolphins died during the observed fishing activity; scaling this up from this fishing subset to the entire fishing fleet, as many as 60 dolphins may die in nets each year. A stranding network has recently been set up around the Balearic Islands, which should help document these fisheries interactions to determine actual fishing-related mortality rates for these dolphins. 

   

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