Removing the big fish from a population leaves the remaining fish more vulnerable to environmental change.
Credit: NOAA
SYDNEY: Going after the big fish could leave species more vulnerable to environmental change, according to a new international study.
Published this week in the British journal Nature, the study suggests that the strategy of taking the bigger, older fish out of a population - commonly cited as a sustainable fishing practice - in fact makes the population more unstable, and more likely to go extinct.
The study analysed the results of a 50-year investigation into the effects of commercial fishing on a wide variety of fish species off the coast of California. While it has long been known that fishing contributes to declining fish populations, this study resolves a long-running dispute over whether commercial exploitation also causes increased variability in fish numbers.
When a fish population fluctuates greatly in size, it has a greater risk of extinction and is harder to manage successfully, according to co-author George Sugihara of the University of California in San Diego. "If fishing results in both higher variability and declining populations, fisheries are in double jeopardy."
"All fish, including those that are not commercially harvested have ... ups and downs in response to natural changes in the environment. What we found is that these relative ups and downs are amplified in commercially fished populations," said Sugihara.
The authors think that the increased variability in exploited fish populations is due to the practice of fisheries selectively targeting bigger, older fish.
According to Sugihara, small numbers of large fish are much better at surviving environmental fluctuations - like the changes caused by El Nino - than are larger numbers of little fish.
"In a year with a lot of food available, 500 little fish can grow much more than five big fish," he said. If the next year is not so good, those 500 little fish - which are now much bigger - might run out of food and starve to death.
Sugihara explained that this is why it's important to maintain a good mix of big and little fish in the population. A population consisting of only little fish can "boom and bust," he said.
"Currently fisheries are managed in terms of specifying a 'total tonnage' for the catch," according to Sugihara. "Our results suggest that some attention should also be given to not just 'how much is harvested' but also to 'who is harvested.'"
The study examined the findings of the California Cooperative Oceanic Fisheries Investigation (CalCOFI), a long-term research project begun in 1951 to investigate the reasons behind the collapse of the California sardine fishery.
"The CalCOFI data are an absolute jewel," said Sugihara. While most information on fish stocks is based on catch data from fishermen - which is only available for exploited species - the CalCOFI data are derived from fish larvae counts across all species in an area.
According to Sugihara, counting fish larvae is an accurate method of working out how many adult fish of a certain species are out there, whether the species is commercially exploited or not.
Using the larvae data, the researchers compared population variables for exploited species with non-exploited species in the same year. This technique enabled the team to filter out environmental factors, and zero in on fishing effects.
