The thinking that fishing is increasing fish populations is certain on short term, but not true long term. A high spike on prey fish means less of their prey, subsequently, therefore diminishing their populations over time.
I would tentatively guess that this doesnāt usually fully reverse the effect on prey fish, only dampens and slightly reverses it, so that their populations still settle higher than without fishing their predators. Furthermore, the food (mostly primary production?) of the prey (mostly crustaceans) of the prey fish should increase, too, which could have the opposite effects. I guess this can lead to algal blooms sometimes, though, which could then reduce all local animal populations.
That being said, I worry about this reasoning anyway, because it treats food webs as quite linear. A species X can eat a species Y and the prey Z of Y.
There are a few studies on the effect of removing a predator from their ecosystem resulting not on increase of fish populations or biodiversity, but in fact, the opposite happening: less biodiversity, lower populations, ecosystem collapse. This can be easily noticed in scenarios before and after implementing a Marine Protected AreaāI can share a few studies if this helps, let me know.
By āremoving a predator from their ecosystemā, do you mean the (near-)complete removal and therefore (near-complete) absence of the predator, or just a reduction in their biomass/āpopulations? The latter seems more representative of fishing to me, especially as management has improved, and local extinction of a predator seems rarer (although it definitely has happened).
Iād be interested in seeing these studies, especially any globally representative aggregates, systematic reviews or meta-analyses to avoid selection bias.
Iāll note thatChristensen et al., 2014is a global aggregate (and extrapolation) of simulations (Ecopath models) spanning 100 years (1910-2010), andBell et al., 2018is a meta-analysis of observational studies of biomass data across trophic levels, each spanning at least 18 years, and with a mean length of 34 years.
Fishing pressure seems to have increased a lot around 1970, and predator biomass had been decreasing much faster since around then, according toChristensen et al., 2014(Table 3 and Figure 6). So, most of the changes to prey fish biomass should be since around 1970, too. 40 years (2010-1970) seems like it should have been long enough to see reversal in trends for prey fish from feedback on their prey, but the net effect was still an increase in prey fish biomass. That being said, I donāt know if the simulationsChristensen et al., 2014used in fact simulated the effects of prey fish on their prey and feedback from that, although Iād guess they did.
Iād guess 18 years inBell et al., 2018is long enough to see feedback from the prey of prey fish, too. We actually do see that many of the increasing trends in small fish populations (in red) reverse into decreasing trends inFigure 5inBell et al., 2018, but the trends vary substantially by survey/āregion and the reversals also often seems to coincide with reductions in fishing pressure (FPI, the background colour going from grey to lighter grey or white; this was the case for S St Lawrence, GSO-Fox Island, GSO-Whale Rock, Georges Bank, Mid-Atlantic), so thereās some confounding here to worry about.
Hi Nathalie, on point 3
I would tentatively guess that this doesnāt usually fully reverse the effect on prey fish, only dampens and slightly reverses it, so that their populations still settle higher than without fishing their predators. Furthermore, the food (mostly primary production?) of the prey (mostly crustaceans) of the prey fish should increase, too, which could have the opposite effects. I guess this can lead to algal blooms sometimes, though, which could then reduce all local animal populations.
That being said, I worry about this reasoning anyway, because it treats food webs as quite linear. A species X can eat a species Y and the prey Z of Y.
By āremoving a predator from their ecosystemā, do you mean the (near-)complete removal and therefore (near-complete) absence of the predator, or just a reduction in their biomass/āpopulations? The latter seems more representative of fishing to me, especially as management has improved, and local extinction of a predator seems rarer (although it definitely has happened).
Iād be interested in seeing these studies, especially any globally representative aggregates, systematic reviews or meta-analyses to avoid selection bias.
Iāll note that Christensen et al., 2014 is a global aggregate (and extrapolation) of simulations (Ecopath models) spanning 100 years (1910-2010), and Bell et al., 2018 is a meta-analysis of observational studies of biomass data across trophic levels, each spanning at least 18 years, and with a mean length of 34 years.
Fishing pressure seems to have increased a lot around 1970, and predator biomass had been decreasing much faster since around then, according to Christensen et al., 2014 (Table 3 and Figure 6). So, most of the changes to prey fish biomass should be since around 1970, too. 40 years (2010-1970) seems like it should have been long enough to see reversal in trends for prey fish from feedback on their prey, but the net effect was still an increase in prey fish biomass. That being said, I donāt know if the simulations Christensen et al., 2014 used in fact simulated the effects of prey fish on their prey and feedback from that, although Iād guess they did.
Iād guess 18 years in Bell et al., 2018 is long enough to see feedback from the prey of prey fish, too. We actually do see that many of the increasing trends in small fish populations (in red) reverse into decreasing trends in Figure 5 in Bell et al., 2018, but the trends vary substantially by survey/āregion and the reversals also often seems to coincide with reductions in fishing pressure (FPI, the background colour going from grey to lighter grey or white; this was the case for S St Lawrence, GSO-Fox Island, GSO-Whale Rock, Georges Bank, Mid-Atlantic), so thereās some confounding here to worry about.