Alternative stable states in marine food-webs – preventing recovery of overexploited fish stocks?

Background/Question/Methods

The potential occurrence of alternative stable states, ASS, (i.e. the presence of hysteresis) in marine food-webs is receiving increasing attention. If present, ASS have important consequences for how to manage exploitation of the oceans. For example, by preventing recovery of e.g. overexploited top piscivores following reduced exploitation rates.  While abrupt changes in species composition have occurred in marine food-webs world-wide, it remains to be demonstrated whether these constitute hysteresis. In several systems, a number of species interactions have been suggested as mechanisms with potential to cause ASS, leading to alternative states with and without top-piscivorous fish. These include mixed interactions, such as intra-guild predation, leading to so called cultivation-depensation effects, and mutual predation, where prey fish predate on early life-stages of top-piscivores, as well as predation-induced changes in prey growth, resulting in poor prey condition or prey population structures with fewer individuals vulnerable to predation. But do these mechanisms occur in exploited marine food-webs? Here we use theory on size-dependent community dynamics to derive a data analysis framework to identify the presence of these mechanisms leading to ASS with and without top-piscivorous fish in natural exploited systems.

Results/Conclusions

We present the data analysis framework in the form of a decision-tree, which based on results from analyses of piscivore and prey performance and abundances, distinguishes between ASS caused by cultivation-depensation, mutual predation, and indirect predation effects. Specifically, we show the piscivore growth patterns, changes in vital rates and abundance, and in biomass and performance of their fish prey and lower trophic level preys, that are consistent with each type of ASS mechanism. Finally, we apply the data analysis framework on the case of Baltic Sea cod, and illustrate each analysis step in the decision-tree. Preliminary results show shifts in growth of both non-piscivorous and piscivorous stages of cod, suggesting the presence of more than one of the ASS mechanisms. In conclusion, predictions from physiologically based model studies of community dynamics can be used to identify the presence of ASS mechanisms in natural exploited food-webs (affecting the potential for piscivore recoveries), but require detailed data on the performance and abundance of piscivores in different ontogenetic stages and of their respective preys.