Fishery selection across the millennia.

Humans have been exploiting the Earth’s resources for ages. Owing to tool limitations and risk of injury (including death), early human hunters targeted those individuals that were relatively easy to kill. On land, for instance, human hunters selectively targeted prey in poor condition, that is, the weak, sick, young and old. In the ocean, early humans selectively targeted accessible prey, that is, those found close to shore. Whether early fishing in the marine environment was also selective with regards to individual phenotypic traits (e.g. size) has been less clear, but new evidence suggests that it was. This issue of Proc. R. Soc. B contains a paper comparing the characteristics of Baltic Sea cod harvested during the Neolithic period (4500 BP) to cod harvested in today’s highly industrialized fishery (Limburg et al. 2008). This paper reports on the results of an interdisciplinary effort bringing together fisheries’ scientists, ecologists and archaeologists to better understand the history of cod exploitation in the Baltic Sea. Such historical analyses are exceedingly rare but provide unique insights into the role of humans as agents of disturbance and selection, which would not be revealed from the analyses of modern data alone (e.g. Jackson et al. 2001). Analysing otoliths (ear stones) and vertebrae collected from the historic and modern periods in the central Baltic Sea, Limburg et al. (2008) estimate both the age and size distribution of cod then and now. The historic otoliths were excavated from a single site (Ajvide) on the island of Gotland, Sweden; the modern otoliths were obtained from fishing areas near Gotland. Based on the archaeological findings of fish hooks and birch bark floats also recovered at the excavation sites, the Ajvide fishery consisted of a combination of hook-and-line (size-selective) and net fishing from shore (non-selective). By contrast, the modern fishery consists of gillnet and trawl fisheries, both of which are known to be highly size selective. Interestingly, only a narrow range of lengths were represented in the Neolithic samples relative to the modern samples (very small and very large cod were notably absent from their Neolithic samples; see also Olson et al. 2008), suggesting that the Neolithic cod fishery indeed was size biased. Another difference between the two periods is the spatial location of fishing—the Neolithic fishery was confined to areas where the fish were accessible (nearshore environment), whereas the modern fishery occurs where harvestable biomass is the greatest (offshore environment). This difference is important because cod are known to move into deeper waters as they grow larger/older (Tomkiewicz et al. 1998). This implies that the Neolithic people at Ajvide did not have access to the largest cod, whereas the modern fishers do (figure 1). Despite this difference, cod caught at Ajvide in the Neolithic fishery were larger and older, on average, than