Life history change in commercially exploited fish stocks: an analysis of trends across studies

Abstract

Age and size at maturation have declined dramatically in many commercial fish stocks over the past few decades - changes that have been widely attributed to fishing pressure. We performed an analysis of such trends across multiple studies, to test for the consistency of life history changes under fishing, and for their association with the intensity of exploitation (fishing mortality rate). We analyzed 143 time series from 37 commercial fish stocks, the majority of which originated from the North Atlantic. Rates of phenotypic change were calculated for two traditional maturation indices (length and age at 50% maturity), as well as for probabilistic maturation reaction norms (PMRNs). We found that all three indices declined in heavily exploited populations, and at a rate that was strongly correlated with the intensity of fishing (for length at 50% maturity and PMRNs). These results support previous assertions that fishing pressure is playing a major role in the life history changes observed in commercial fish stocks. Rates of change were as strong for PMRNs as for age and size at 50% maturity, which is consistent with the hypothesis that fishing-induced phenotypic changes can sometimes have a genetic basis.

Keywords: contemporary evolution; darwins; fisheries-induced evolution; life history evolution; microevolution; over-fishing; rapid evolution; selective harvesting.

Figure 1

Magnitude of phenotypic change in response to fishing mortality for length at 50% maturity (A,B), age at 50% maturity (C,D) and mid-points of probabilistic maturation reaction norms (PMRNs) (E,F). The Y axis shows residuals from a linear regression of darwin numerators ([ln(Z₁) – ln(Z₀)]) over time (log10 years); i.e., proportional phenotypic change after accounting for the effects of time. Fishing mortality is the average of yearly estimates of fishing mortality for the time period over which the phenotypic change was measured. Note that one data point (F = 1.9) is not shown in panels C (x = 0.19, y = −0.10) and D (x = 0.19, y = −0.18) so as to match the scale in the other panels. Trendlines were fit only in cases where fishing mortality was found to be significant (P < 0.05).

Figure 2

Rates of phenotypic change for stocks experiencing low (F < 0.3), medium (0.3 ≤ F < 0.6) and high (F ≥ 0.6) levels of fishing mortality (year⁻¹). Rates are expressed in darwins (×10³) and are plotted separately for length at maturity (A), age at maturity (B) and midpoints of Probabilistic Maturation Reaction Norms (PMRNs) (C). The thick lines represent the median of each distribution, while the top and bottom of the boxes represent the 75th and 25th percentiles, respectively. The dashed error bars represent 1.5 times the interquartile range (approximately 2 standard deviations). Outliers are shown as separate data points.