Trends in the size and age structure of marine fishes

Size-selective harvesting is expected to reduce the average age and weight of commercially exploited fishes. The loss of larger, older fish has been hypothesized to negatively affect metrics of population viability, such as spawning behaviour, recruitment, and adult survival. Most studies to date have focussed on individual stocks. Here, we examine trends in average age and weight at broad taxonomic and temporal scales, using subsets of data compiled on 95 marine fish stocks. Following moderate declines between 1960 and 1990, we find that the average age has generally increased since 2000, such that 71% of 69 stocks are currently above their long-term average. However, the size of the oldest individuals has generally declined over time; the average weight is currently below average in 75% of 55 stocks. A temporal decline in the mean weight of the youngest constituents within 49 stocks is most evident in the Clupeiformes. Our results indicate that recovery of age structure need not be accompanied by recovery of weights-at-age, evidenced in part by a decline in the size of the oldest individuals within populations. Further study into the drivers of these patterns, and the consequences of declining weights-at-age for population viability, is warranted.