By many accounts, humans have dramatically reduced the number of fish in the sea. About 15 years ago, Myers and Worm estimated that global fish stocks of the most desirable predatory fishes, like red snapper and swordfish, were now at only 10% of the levels of the preindustrial era. Recreational fishers can win a local tournament with a fish that is only a fraction of the size that it took to win several decades ago. And, the best restaurants serve fish that were considered trash fish a century ago. All this constitutes what Daniel Pauly calls “fishing down” aquatic food webs. We must be happy with fewer fish, smaller fish, and less desirable fish than 100 years ago.
With the exception of the lobster fishery, most fishing regulations suggest that we throw back the small ones, with the idea that they will grow up to larger, more desirable fishes. This is at odds with recent data showing that the reproductive output of fishes increases disproportionately with body size. Big mothers put out many more eggs than their smaller daughters and granddaughters. For American cod, one large female has the reproductive output of 37 smaller females, who collectively will weigh 2.5 times as much. We should be throwing back the large fish as breeders and keeping the smaller individuals to eat.
This would represent a dramatic change in most current policies designed to protect and achieve sustainable levels for fisheries. A policy of throwing back the large fish might not be easy to implement, when tradition has promoted using nets with openings designed to let the small fish pass through. And small fish are less pleasing to the recreational angler. Nevertheless, as the ocean’s stocks of fishes have been depleted under existing policy, some dramatic new changes are needed.
These changes in policy are made more critical by observations that most marine fishes are likely to have lower reproductive output and success in a warmer climate, already manifest in warming ocean temperatures recorded by the Navy.
When the world’s rising human population demands more food from the sea, we must take drastic steps to ensure the sustainability of the marine ecosystem for future generations.
References
Barneche, D.R., et al. 2018. Fish reproductive-energy output increases disproportionately with body size. Science 360: 642-645.
Frank, K.T., et al. 2005. Trophic cascades in a formerly cod-dominated ecosystem. Science 308: 1621-1623.
Free, C.M., et al. 2019. Impacts of historical warming on marine fisheries production. Science 363: 979-983.
Myers, R.A. and B. Worm. 2003. Rapid worldwide depletion of predatory fish communities. Nature 423: 280-283.
Pauly, D., et al. 2000. Fishing down aquatic food webs. American Scientist 88: 46-51.
Shackell, N., K. Frank, J. Fisher, B. Petrie and W. Leggett. 2009. Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem. Proceedings of the Royal Society B: Biological Sciences 277: 1353-1360. doi: 10.1098/rspb.2009.1020
By keeping/eating the larger fish and returning the smaller fish to the water we humans have been changing the genetics of the fish. Normally in animal or plant breeding, a person would save the larger for continued breeding, and cull (eat) the smaller. We are witnessing the result of dis-genic selection, selecting and breeding for smaller fish.
Fascinating observations! I’m curious whether you think a shift in fisheries management toward incentivizing capture of smaller fish would even be sufficient to maintain current stocks or, ideally, to ultimately replenish them to something approaching pre-industrial levels in the face of relentless global increase in demand for seafood (coupled with the reproductive losses from warming, as you mention). All of this sometimes feels like water under the bridge to me. I work for an organization that promotes plant-based and cell-based seafood as truly sustainable options that don’t require consumers to settle for less desirable species or lower consumption levels, but it seems this approach isn’t yet widely acknowledged among advocates for sustainable fisheries management as a complementary strategy to their work.
Thanks for your blog!
While a policy of harvesting the smaller fish and letting larger, reproductive adults live to go on and breed might make biological sense, it would, as you say, be difficult to implement, particularly at this point in fisheries (over)development. But I’m glad to see you bringing up the issue! From what I’ve been reading, it looks like the best hope for allowing fish populations to regenerate will be establishing large marine protected areas and closing off other areas in strategic locations to fishing; on the open oceans, however, the RMFOs, largely representing powerful commercial fishing fleets, may act to oppose this move–even though researchers have predicted that protecting large breeding areas would likely increase the harvest for everyone. Daniel Pauly discussed this in an interview last year (https://e360.yale.edu/features/a-global-ban-on-fishing-on-the-high-seas-the-time-is-now); seamounts especially should be protected (Watling and Auster 2017; https://www.frontiersin.org/articles/236096).
While throwing back the big fish may seem like a good idea, there are several reasons it won’t work. The trawl fishery (eg. cod) so damages the fish that it is unlikely many would survive if released. Trawling also does significant damage to the benthic infauna. In serranids (groupers and their allies) the larger fish are for the most part males and thus would not protect many females. Further, decompression from the depths they are captured causes significant tissue damage. The Monterrey Bay guys developed a method to return fish to depths but it is a labor intensive activity. So the only real hope is to establish no fishing zones.