Fishery management tries to balance multiple fisheries objectives, including catches (i.e. food production), industry profit, and species conservation--both targeted and not targeted (called 'bycatch'). Sometimes these objectives are aligned, meaning 'win-win' management strategies exist that simultaneously improve the fishery with respect to each objective. Other times there are tradeoffs, and management has to weigh various objectives against each other.
Conventional wisdom suggests that rebuilding an overfished target population is a win-win solution: a healthier fish population can sustainably support higher catches and a more profitable fishery. For instance, in the U.S. Atlantic sea scallop fishery, management changes in 1994 led to large increases in both catches and the scallop population size over the next decade. Conventional wisdom also suggests that protecting endangered bycatch species, such as sea turtles, commonly introduces a tradeoff with maximizing sustainable catches or profits from the target species. The Hawai'i longline swordfish fishery is an interesting example of this. It has been closed several times over the past 20 years due to excessive sea turtle bycatch, despite having a stable and healthy target population (north Pacific swordfish).
My colleagues and I have recently uncovered several situations that seem to run counter to this conventional wisdom.
In a paper published last year in Science with Grant McDermott and several others from emLab, we examined 20 declining marine mammal, turtle, and seabird populations threatened as bycatch. We projected that roughly half of these populations would no longer be in decline if fisheries were managed to maximize profits from their target species. In other words, these are cases in which the declining bycatch population is a side-effect of overfishing the target population, meaning rebuilding the target population would not only increase fishery profits, it would also prevent further declines in the bycatch population. This is a win-win management solution. We found that win-win opportunities were most available for sea turtles, and in the coastal waters of developing countries where there is more overfishing.
A 2017 study led by my fellow emLab alumnus Cody Szuwalski showed that rebuilding target populations in the East China Sea using western-style, single-species management might substantially decrease catches at the ecosystem level, rather than increase them, as conventional wisdom would suggest. The explanation for this outcome is the food chain--decades of intense and indiscriminate fishing have largely depleted the East China Sea of its largest fish. As a result, smaller fish are free from predators, and more are available to catch than would be normally. If management rebuilt the predator populations, the fishery would be able to catch more predators but fewer prey, but the loss in prey catches would far outweigh the gain in predator catches due to the inefficiency of energy transfer in food chains.
To draw an analogy, imagine if we stopped keeping deer out of wheat fields. The biomass of deer we would gain on the fields would be far smaller than the biomass of wheat we would lose to their grazing.
Another 2017 study led by my colleague Nis Jacobsen from University of Washington found similar tradeoffs between ecosystem-level catches and target species abundance in three other large marine ecosystems. However, it also suggested that increasing target-species abundance was largely aligned with increasing fishing profits at the ecosystem level, similar to the conventional wisdom. Why this difference between catches and profit? Larger fish tend to receive higher market prices (per pound), which counteracts the effects of food-chain inefficiency, which makes predators relatively low-yielding, in terms of catch.
What does all of this mean for fishery management?
1) We should be cautious about exporting management strategies that have worked well in developed countries to developing countries. Several developing countries--in Asia and elsewhere--have decades-long histories of intense indiscriminate fishing, similar to the East China Sea. They also have human populations that are highly-dependent on fish as a food source. Our work raises the possibility that western-style, single-species management could decrease catches and exacerbate food insecurity in these regions. Ecosystem-level management approaches would likely produce better outcomes. The challenge will be making such approaches workable with limited data--a challenge whose solutions I am currently exploring in collaboration with emLab.
2) At the ecosystem level, there may be more tradeoffs between fishery catches and fishery profits than we previously thought. This means managers will need to be more deliberate in weighing these objectives against each other--as well as food production alternatives, such as aquaculture--rather than assuming they are aligned.
3) If the objectives of protecting endangered bycatch species and maximizing fishery profits are closely aligned, then management interventions aimed at either objective could result in improvements for both. For instance, a new U.S. trade rule may force several countries to improve their monitoring and protection of marine mammals caught as bycatch in their fisheries. Our work suggests that some of these interventions could have positive side-effects on local fisheries. Similarly, efforts to rebuild overfished target populations are likely to have positive side-effects on bycatch populations.
