Low tolerance of salt water in a marine fish: new and historical evidence for surprising local adaption in the well-studied commercially exploited capelin

Abstract

Intraspecific biodiversity among populations is ecologically and evolutionarily notable. Substantial adaptations in well-studied commercial species should be obvious and ought to have been uncovered. The tenet of marine stability posits relatively little local adaptation in the sea, except for temperature responses to latitude for widespread species. However, aquatic reproduction is tightly constrained by water chemistry, which may lead to abstruse adaptations. Capelin (Mallotus villosus) reproduction has been studied for over a century, occurring within pebble substrate on beaches or the seafloor of continental shelves. Offshore spawners (Iceland/Barents Sea) have embryos with tolerance of high salinity. I provide new and historical data on beach spawners from several areas indicating that their embryos perform well from ∼2 to 28 psu, but at higher salinities indicative of coastal seawater, there is poor hatch success, larvae take longer to hatch, hatch at a smaller size, and starve more quickly. The body of evidence supports the hypothesis that beach spawning evolved from anadromy, with offshore spawning a derived state, enabled by increased salinity tolerance. The results have recovery implications for depleted Newfoundland stocks, which have been spawning seasonally late under relatively high salinity for over 25 years.