Abstract
Many severely depleted populations of baleen whales (Mysticeti) have exhibited clear signs of recovery whereas there are few examples in toothed whales (Odontoceti). We hypothesize that this difference is due, at least in part, to social and behavioural factors. Clearly, a part of the lack of resilience to exploitation is explained by odontocete life history. However, an additional factor may be the highly social nature of many odontocetes in which survival and reproductive success may depend on: (a) social cohesion and organization, (b) mutual defence against predators and possible alloparental care, (c) inter-generational transfer of “knowledge”, and (d) leadership by older individuals. We found little evidence of strong recovery in any of the depleted populations examined. Their relatively low potential rates of increase mean that odontocete populations can be over-exploited with take rates of only a few percent per year. Exploitation can have effects beyond the dynamics of individual removals. Four species showed evidence of a decrease in birth rates following exploitation; potential mechanisms include a deficit of adult females, a deficit of adult males, and disruption of mating systems. The evidence for a lack of strong recovery in heavily exploited odontocete populations indicates that management should be more precautionary.
Highlights
In a 1993 paper, Best [1] examined increase rates in severely depleted stocks of baleen whales (Mysticeti)
At least three killer whale populations have not recovered as expected, and the same is true of two dolphin populations in the eastern tropical Pacific
We are the first to admit that the data summarised in this paper are only suggestive and that firm conclusions linking social and behavioural factors to nonrecovery or compromised recovery of odontocete populations would be premature
Summary
In a 1993 paper, Best [1] examined increase rates in severely depleted stocks of baleen whales (Mysticeti). In many populations there are examples of substantial seasonal movements in search of prey, such as movements following herring out of fjords in Norway into the Atlantic Ocean These characteristics make killer whales, like the other species described above, potentially vulnerable to social disruption and the loss of cultural and ecological knowledge through removal of older individuals. Survival rates in the northern resident population have been correlated with salmon abundance [63], and northern residents have high levels of POPs, so the same confounding factors apply to this population when it comes to explaining the lack of sustained recovery Individuals from both resident and “transient”-type (mammal-eating) populations of killer whales in Alaska apparently died from exposure to oil in the Exxon Valdez spill and suffered losses of 33% and 41%, respectively, in the year following the event [69]. This may make pilot whales vulnerable to the removal of certain individuals from social groups
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