Broken symmetry of recruitment fluctuations in marine fishes: L\'evy-stable laws and beyond

TL;DR

This paper investigates how heavy-tailed reproductive variability influences fish recruitment fluctuations, revealing that individual reproductive success significantly impacts aggregate population measures, especially under environmental variability, with evidence from North Atlantic fish stocks.

Contribution

It demonstrates that heavy-tailed offspring distributions cause asymmetric, Levy-stable fluctuations in fish recruitment, highlighting the importance of individual reproductive variability in population dynamics.

Findings

Recruitment fluctuations follow Levy-stable laws with heavy tails.

Reproductive variability dominates over environmental effects in fluctuations.

Recruitment shows asymmetric, right-skewed fluctuations indicating idiosyncratic reproductive success.

Abstract

Recruitment is calculated by summing random offspring-numbers entering the population, where the number of summands (i.e. spawning population size) is also a random process. A priori, it is not clear that individual reproductive variability would have a significant impact on aggregate measures for monitoring populations. Usually these variations are averaged out in a large population, and the aggregate output is merely influenced by population-wide environmental disturbances such as climate and fisheries. However, such arguments break down if the distribution of the individual offspring numbers is heavy-tailed. In a world with power-law offspring-number distribution with exponent $1<\alpha<2$, the recruitment distribution has a putative power-law regime in the tail with the same $\alpha$. The question is to what extent individual reproductive variability can have a noticeable impact on the recruitment under environmentally driven population fluctuations. This question is answered by considering the L\'evy-stable fluctuations as embedded in a randomly varying environment. I report fluctuation scaling and asymmetric fluctuations in recruitment of commercially exploited fish stocks throughout the North Atlantic. The linear scaling of recruitment standard deviation with recruitment level implies that the individual reproductive variability is dominated by population fluctuations. The totally asymmetric (skewed to the right) character is a sign of idiosyncratic variation in reproductive success.