After a collapse: long-term effects of biomass reduction

The worldwide human footprint is increasingly evident. Over the decades, many species have collapsed or are collapsing with only some recovering. A key question is what happens to these populations after the collapse. In a study published in Scientific Reports (Durant et al 2024), we test whether the effect on fish populations from overfishing, climate, and survival of young fish remains the same after a collapse.

colour figure explaining the choice of stocks, red is at the bottom for biomass reduction indicating a collapse

Figure 1: Overview of the data selection to describe a “collapsed/recovered” fish stocks. The objective is to evaluate the consequences of large population biomass decline on the sensitivity to external and internal drivers in marine fishes. The working hypothesis is that stocks that have experienced a collapse in biomass may respond differently to external drivers post-collapse than they did pre-collapse.

Traditionally, marine fish collapses were linked to overfishing, poor climate, and recruitment (survival to large enough size to be fished). Overfishing and climate change have been studied extensively in relation to fish population collapses, particularly for economically important species. However, the persisting dynamical changes resulting from an important biomass reduction in fish populations are less well studied. Durant et al (2024) investigate if biomass collapse affects a stocks response to external drivers even after some level of recovery.

**Figure 2:** General location of the 76 fish stocks with collapse used in the study of Durant et al (2024). Most stocks in our study are from the northern hemisphere (Europe, North America). The 54 retained stocks after modelling (blue dots) correspond to 42 fish species. Among these, 20 were demersal fishes, 6 pelagic fishes, 3 benthopelagic fishes, 4 bathydemersal fishes, 1 bathypelagic fish, and 8 reef associated fishes. Note that several stocks are on the same location.

By analysing long-term data extracted from the RAM Legacy Stock Assessment Database, the study the effects of fishing, sea temperature, and recruitment on the change in biomass (i.e., the total mass of all fish of the considered species for the considered stock).

Comparing the pre-collapse statistical model to the post-collapse one on 54 recovering exploited fish stocks, representing 42 different species in various habitats, Durant et al. (2024) provided a comprehensive understanding of their respective roles in shaping population dynamics.

For most stocks, the fishing effect became weaker after the collapse. The strongest signals were found for pelagic fishes. While biomass may have been rebuilt, the sensitivity to fishing, sea temperature, and recruitment did change indicating a deeper effect of collapse on stock than solely a reduction in fish numbers.

This study also show that a general model applied to many stocks may provide useful insights, but since not all stocks respond similarly to a collapse, stock-specific models may give more detailed insights.

References:

Durant JM, Holt RE and Langangen Ø (2024) Large biomass reduction effect on the relative role of climate, fishing, and recruitment on fish population dynamics. Scientific Reports, 14: 8995.

Tags: RAM legacy, Climate change, overfishing, Population dynamics By Joel Durant, Øystein Langangen