Fisheries often preferentially target large fish over small ones. Once stocks become overexploited, their size structure tends to shift towards smaller individuals. This also means that the selection pressure shifts from one where it is favourable to be large (producing many offspring and not being so readily eaten) to one where it is instead favourable to mature and spawn earlier, thereby increasing the probability of having offspring under increased fishing pressure. To do this, fish typically accelerate growth at juvenile ages and decelerate growth at mature ages which has been shown to sometimes have an evolutionary basis.
Studying the size and growth of fish in the past could tell us something about how size-selective modern fisheries are, when exploitation intensified, and what impact this (and changing environments in general) has had on the ecology and evolution of fish. This information is lacking due to difficulties in obtaining long-term biological data from wild populations. However, it is incredibly important to effectively forecast population dynamics and support sustainable fisheries management since fish size and growth affect many metrics used to assess stocks, such as size at maturation, fecundity, recruitment, and biomass.
The authors of the latest Editor's Choice article in ICES Journal of Marine Science use archaeological, archived, and modern vertebrae to reconstruct the historical catch-at-size and early-life growth of Atlantic bluefin tuna (Thunnus thynnus) over two millennia in the eastern Atlantic and Mediterranean. Novel evidence demonstrates that tuna juvenile growth increased between the 16th–18th, 20th, and 21st centuries, and correlates with a warming climate and a likely decrease in stock biomass. The authors found it equally plausible that fisheries-induced evolution has acted to increase juvenile tuna growth, driving earlier maturation as a result of size-selective exploitation.
Coincidently, the authors found limited evidence to suggest a long history of large (>200 cm fork length) tuna capture. Instead, they found that the catch-at-size of archaeological tuna since the third century BCE was relatively small in comparison with the more intensive tuna trap fisheries which operated further from shore from around the 1900s onwards.
The authors call for further studies to assess the onset of intensive exploitation on marine populations and disentangle the effects of biomass declines, climate change and size-selective exploitation on fish growth. For research (or fun!), visit the webpage used to reconstruct archaeological tuna size.
Read the full paper, Vertebrae reveal industrial-era increases in Atlantic bluefin tuna catch-at-size and juvenile growth, in ICES Journal of Marine Science.
