Can management measures be used to reduce the impact of extreme events in the early life of fish?

As we are experiencing in many parts of the world, extreme events are happening more often. Extreme climatic events, (e.g., heat waves and storms) are becoming more frequent and intense, and on top of that, the ever-growing development of human activities at sea entails an increasing risk of acute events (e.g., oil spills). Those extreme events can affect a whole range of living organisms potentially leading to mass mortality events.

In a recent study, Buttay et al. (2023), have focused on perturbations that could impact the early life stage of fish. Taking the Northeast Arctic (NEA) cod as an example, they evaluated different mitigation strategies to alleviate the effect of such dramatic events on the population.

The early life stage of fish, which here refers to the eggs and larval stages, represents a short time within the life of a fish, but it is often a critical period in which individuals are highly sensitive to perturbations. Survival through this period typically plays a key role in the dynamics of fish populations. Extreme events occurring during the early life stages of fish could lead to a mass mortality event. In a previous work, two of the co-authors of the present study, investigated what would be the impact for the cod population of a 50 % or 99 % mass mortality event on eggs and larvae (Langangen et al. 2023; post).

Their results show that the population is generally resilient to such catastrophic events. The effects were moderate because of density dependence in survival in later life stages and due to the broad population age structure, which buffers the loss of a single-year class. Although population recovery is relatively fast (less than 15 years), losses in harvest and economic value can nonetheless be substantial.

Young age classes of cod are not targeted by fishing. Indeed, cod recruits to the fishery at the age of 3 years. Consequently, the effect of a potential mass mortality event on eggs and larvae would only be visible in the fishery after approximately 3–6 years. As such, before the cohort potentially weakened by the mass mortality event recruits into the fishery, there is a window of opportunity to implement management actions to alleviate potential long-term effects on catches and population abundance.

The nature and timing of extreme events are often unpredictable. By using numerical models and simulating extreme events, it is however possible to evaluate possible management options.

Can we, by reducing fishing during some years, alleviate the effect of a mass mortality event affecting eggs and larvae? How much and how long should we reduce fishing?

In this study, a life cycle model for the NEA cod population is used (Figure 1) to assess the efficiency of different fishing reductions. This model is fitted to observational data (e.g., stock assessment biomass, landing, weight at age etc.) and reproduces the past dynamic of NEA cod from 1959 to 2012. To test the efficiency of different mitigation strategies, mass mortality events on eggs and larvae (50 or 99% mortality) were simulated followed by fishing reduction of different intensities (10%, 20%, up to 100%) and different duration (from 1 year up to 10 years of fishing reduction). In order to compare the different scenarios between them, the total catch and the inter-annual variability in catch over a period of 10 years was used.

The results of this study indicated that fishing reductions can be used to buffer the effects of mass mortality events. Moreover, a short and intense reduction in fishing (100% reduction in one year) will often lead to increasing catch over a 10-year period but is associated with large variations in catch between years. On the other hand, a moderate fishing reduction applied shortly after the adverse event will lead to increased catches over a ten-year period without a drastic increase in the variability in the annual catches. Importantly, the mitigation efficiency will be higher if the stock is in a healthy shape, as indicated by e.g., a higher mean age of the fish in the population (Figure 1). These findings demonstrate that adaptive management may be beneficial for the management of marine fish.

References

Buttay L., Ohlberger J. & Langangen Ø. (2023) Management strategies can buffer the effect of mass mortality in early life stages of fish. Journal of Applied Ecology doi:10.1111/1365-2664.14489.

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.