In 1951 Bill Ricker and his colleagues at the Pacific Biological Station became interested in the size of the salmon caught in the West Coast fishery (Ricker 1995). By 1975 they had observed that pink salmon (Oncorhynchus gorbuscha), coho (O. kisutch) and chinook (O. tshawytscha) had all declined in size, pink salmon by 40%, coho dropped 20-30%, and chinook from 9kg to 6kg. Since 1975 changes have become more complicated in different parts of the fishery (Ricker 1995). Why these changes? Ricker concluded:
“From the data available up to 1975, I suggested that a change in the genetic constitution of salmon stocks was mainly responsible for the observed decreases in size. After all, if you are raising beef cattle, for example, you select breeding stock with a proven history of fast growth. Our fisheries have been doing exactly the opposite.” (Ricker 1995 page 600).
Everyone had read Charles Darwin and knew about natural selection and here was more clear evidence in the natural world.
About this time wildlife managers became interested in the same possibility that by hunters selecting the largest mammals in order to obtain a trophy catch, there might be changes in the genetics of large mammal populations that could be detrimental. Festa-Bianchet and Mysterud (2018) have now reviewed the literature on the evolutionary pressures from size-selective harvests of large mammals and have shown that a series of strong conditions must be present to determine if hunting is exerting evolutionary pressure within a harvested population. They point out that the problem is far from simple. For a start one must determine if the trait that hunters select for is heritable. Since often this is antler or horn size or other dominance traits, we need to know how heritable such a trait is. For those large mammals for which we have data, heritability is low to moderate (20-40%). Then we need data on the strength of hunting selection in relation to sexual selection for large antlers or horns or body size. In general, the detection of evolutionary changes in natural populations is difficult.
Festa-Bianchet and Mysterud (2018) review the strength of inference from the available data and point out that the gold standard of ‘experimental manipulation with identified genes that affect horn/antler size, and evidence of changes in both gene frequency and trait size after manipulation’ has not been achieved for any species at the present time. Weaker evidence is available from long-term monitoring studies of several populations of the same species that are subject to different hunting pressures, and even these weaker studies are available for only a handful of ungulate species. The bottom line is that we need much more research on this ‘simple problem’ to make sure that hunting is sustainable from both an ecological and an evolutionary viewpoint.
Back to the fisheries. There has been an explosion of interest in the potential effects of fishing methods on changes in fish stocks. Kuparinen and Festa-Bianchet (2017) provide a good overview while Tillotson and Quinn (2018) dig into the details of potential selection by fisheries on the timing of migration and breeding. Morita (2019) leads us back to the Pacific salmon and how we could be selecting for earlier migrations from ocean to fresh water breeding grounds. Clearly there is much left to do on this important ‘simple’ topic.
Festa-Bianchet, M. & Mysterud, A. (2018) Hunting and evolution: theory, evidence, and unknowns. Journal of Mammalogy, 99, 1281-1292. doi: 10.1093/jmammal/gyy138
Kuparinen, A. & Festa-Bianchet, M. (2017) Harvest-induced evolution: insights from aquatic and terrestrial systems. Philosophical Transactions of the Royal Society of London, B, 372, 20160036. doi: 10.1098/rstb.2016.0036
Morita, K. (2019) Earlier migration timing of salmonids: an adaptation to climate change or maladaptation to the fishery? Canadian Journal of Fisheries and Aquatic Sciences, 76, 475-479. doi: 10.1139/cjfas-2018-0078
Ricker, W.E. (1995) Trends in the average size of Pacific salmon in Canadian catches. Climate Change and Northern Fish Populations (ed. R.J. Beamish), pp. 593-602.Canadian Special Publication of Fisheries and Aquatic Sciences, Ottawa, Ontario.
Tillotson, M.D. & Quinn, T.P. (2018) Selection on the timing of migration and breeding: A neglected aspect of fishing-induced evolution and trait change. Fish and Fisheries, 19, 170-181. doi: 10.1111/faf.12248