Killer whales are stealing our fish to make extra babies
Killer whales (Orcinus orca) didn’t get their name because they’re gentle herbivores. They are top marine predators, and as a species they feed on a variety of critters from fish to seabirds to marine mammals—including other whales. They are highly intelligent, long-lived animals, with complex social dynamics and traditions that vary from group to group. In other words, they have culture. And each family – orca societies are organized according to maternal relatedness – has its own customs and ways of surviving. Those customs are passed from individual to individual, much like human culture.
Over the past 50 years, even as whaling itself has been banned in most of the world, overfishing has impacted killer whale populations. That’s because the availability of prey is a critical factor in determining the long-term population viability of apex predators like killer whales. In other words, more food means more babies. Most females begin reproducing around age 10, and when resources are abundant, give birth to a calf approximately every 5-6 years until they go through menopause. Even after they stop reproducing, it’s thought that female orcas can live to more than 90 years of age.
But overfishing isn’t the only way that humans can impact killer whale populations. It turns out that fishing itself can influence their reproductive output, and sometimes for the better. Because it’s even better – for the predators – when they don’t have to work that hard to actually hunt their food. Instead, they steal it.
That’s what the killer whales off of the Crozet Islands, an archipelago in the Southern Ocean between Africa and Antarctica, are doing at least. Prior to 1996, the killer whales there were on their own, but that year a major Patagonian toothfish fishery was established in that area. Eventually, some groups of killer whales learned to snatch fish off of longlines. Because the groups that learned to eat those fish had a virtually endless supply of food that required no effort to catch, the individuals in those families, theoretically, should have been more reproductively successful than those who had to work for their dinner.
To find out if the theory holds water, a group of French researchers led by Paul Tixier observed groups of longline-thieving orcas and nearby non-thieving orca families for ten years, 2003-2012. That much data allowed them to statistically estimate the reproductive probability of 21 individual females.
As expected they found a positive relationship between reproductive potential and longline depredation. More specifically, those females who feasted upon hooked toothfish were 4% more likely to successfully deliver a calf the following year than those who did not. Four percent might not seem like that much, but for a long-lived species like killer whales, it can add up. While correlations between food availability and fecundity have been observed before for killer whales, Tixier and colleagues believe that theirs is the first study to document a correlation between artificial food sources provided by fisheries, and marine mammal population dynamics.
Over the course of their study, Tixier and colleagues estimated that the Crozet killer whales gobbled up about 17.7% of the total amount of toothfish caught on longlines, which adds up to some 116 tons of biomass each year. In 2012, a single matriline was responsible for more than half of the depredated longlines that year, which meant that they wiped out at least 431,520 hooks and up to ten tons of toothfish for each of the five individuals within that family that year alone.
Even more impressively, killer whales only interacted with longline fisheries an average of two weeks each year, so most of their nutrition comes from other sources. Still, the extra boost of food from longlines seems to allow those pods a slight reproductive advantage.
“The depredation levels reported in the Crozet [exclusive economic zone] are among the largest ever recorded for a similar type of depredation in the world, and cause substantial financial losses to fishing companies,” write the researchers. And those losses could increase as whale populations worldwide continue to recover from a history of whaling, and as their food sources, like fish and pinnipeds, recover as well.
The findings suggests that human activities in the form of longline fisheries have the potential to seriously alter the demographics of long-lived predators like killer whales. When conservation measures and considered, they’re usually meant to cover entire populations, rather than individual social groups. But this research suggests that different matrilines, even within a single population, may have very different life histories and foraging traditions, and therefore may require very different conservation strategies.
Those kinds of patterns ought to be factored in when considering conservation measures for similarly long-lived, highly social predators both there and elsewhere in the world. – Jason G. Goldman | 24 April 2015
Source: Tixier P., M. Authier, N. Gasco & C. Guinet (2014). Influence of artificial food provisioning from fisheries on killer whale reproductive output. Animal Conservation, 18(2) 207-218. DOI: 10.1111/acv.12161.
Header image: shutterstock.com
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