Persistence of fish populations to longer, more intense, and more frequent mass mortality events

Extreme climate events, overexploitation and in general human activities can lead to a strong elevation of mortality, particularly for young and sensitive life stages. Such mass mortality events are predicted to occur more frequently. In our study published in Global Change Biology (Langangen & Durant, 2024), we recorded the chance of collapse of fish populations confronted to various levels and frequency of mass mortality events.

Figure 1. Overview of the modeling approach. The time between each mass mortality event (denoted frequency, red downward pointing arrows), the intensity can vary (length of the arrows) as well as the duration of the events. The biomass trajectories for the perturbed cases (low-intensity mass mortality: dashed line and high-intensity mass mortality: dotted line) are compared with the unperturbed dynamics (solid line) to determine the probability of collapse for different frequency, duration, and intensity of mass mortality.

Recently, it has become clear that extreme events associated with climate warming are becoming more common. Marine heatwaves are an example that has received prominent attention. How fish populations respond to such events is unclear, but one concern is elevated mortality in the early life stages, for example, egg and larval stages. Elevated mortality, often denoted mass mortality events, in early life can also be associated with other stressors, such as marine oil spills and toxic algal blooms. Predicting the frequency and intensity of future mass mortality events in fish is challenging. As an alternative approach, we quantified the tolerance level of fish to increase the frequency and intensity of mortality events (Langangen & Durant, 2024). It has previously been shown that fish are generally quite resistant to single-year mass mortality in early life (see Langangen et al. 2023 and post). In the recent study, we took a scenario approach (Fig.1) and we recorded the probability that the population biomass would be significantly reduced (50% compared to baseline).

**Figure 2.** Probability (color code with numbers) of persistence (not falling below 50% of the baseline) average for all the fish as a function of frequency and intensity of the mass mortality event.

With this scenario approach, we were able to quantify the population level tolerance of 39 different fish species to increased frequency and intensity of mass mortality events. One example of this quantification is shown in Figure 2. We did not find any clear taxonomic signal (different orders did not show clear differences) but we found a clear relationship between the strength of density dependence in early life, longevity, and between-year variation in survival (recruitment variation). Density dependence tends to dampen the mortality events, as the survivors will typically experience less competition. Longevity also tends to dampen effects as it indicates that there are more individuals in cohorts that are not affected by the events. Finally, if there are high variations in early life survival, some very strong year classes may be affected and lead to a high impact of a mortality event. Our predictions are urgently needed for a better understanding of climate effects on fish populations and potentially for mitigation of climate change.

References:

Langangen Ø., Ohlberger J., Stige L.C., Patin R., Buttay L., Stenseth N.C., Ono K. & Durant J.M. (2023). Effects of early life mass mortality events on fish populations. Fish and Fisheries 24(1): 176–186. doi:10.1111/faf.12718.

Langangen Ø. & Durant J.M. (2024). Persistence of fish populations to longer, more intense, and more frequent mass mortality events. Global Change Biology, 30(3), e17251. https://doi.org/10.1111/gcb.17251

Tags: Marine biology, Population dynamics, Fish, Climate change, Heatwaves, Mass-mortality By Øystein Langangen, Joël Durant