Congruent trends in long‐term zooplankton decline in the north‐east Atlantic and basking shark (Cetorhinus maximus) fishery catches off west Ireland

Abstract

Trends in basking shark (Cetorhinus maximus) fishery catches off Achill Island, west Ireland between 1949 and 1975 were examined in relation to zooplankton (total copepod) abundance in four adjacent sea areas over a 27‐year period. The numbers of basking sharks caught and copepod abundance showed downward trends and were positively correlated (r‐value range, 0.44–0.74). A possible explanation for the downward trend in shark catches was that progressively fewer basking sharks occurred there between 1956 and 1975 because fewer copepods, their food resource, occurred near the surface off west Ireland over the same period. We suggest that the decline in basking sharks may have been due to a distributional shift of sharks to more productive areas, rather than a highly philopatric, localized stock that was over‐exploited.

Basking Shark (Cetorhinus maximus) Movements in the Eastern North Pacific Determined Using Satellite Telemetry

“…McFarlane et al (2000), using a composite index, identified a shift in climate ocean conditions that resulted in a decrease in productivity in a range of fish species. Inter-annual regional shifts in the abundance of basking sharks off the U.K. have been linked to zooplankton abundance (Sims and Quayle, 1998;Sims and Reid, 2002;Doherty et al, 2017). The reduced sightings could also be a function of the natural variability or a decline in the population in the ENP (Squire, 1990;Darling and Keogh, 1994;McFarlane et al, 2009).…”

“…Unfortunately, which taxa they were targeting is not known. While copepods are thought to be the preferred prey (Baduini, 1995;Sims and Reid, 2002;Siders et al, 2013;Curtis et al, 2014), stomach contents from basking sharks foraging in deep waters in the Northwest Pacific Oceans included small crustaceans of up to 5.4 cm (Mutoh and Omori, 1978). One factor that suggests the basking shark was targeting copepods is the early ascent prior to dusk, which is unusual for pelagic fish that target the sound-scattering layer (Carey and Robison, 1981;Carey and Scharold, 1990;Musyl et al, 2004;Dewar et al, 2011).…”

Differential contribution of diatoms and dinoflagellates to phytoplankton biomass in the NE Atlantic Ocean and the North Sea

“…Interannual fluctuations in phytoplankton species composition and abundance have received less attention in the NE Atlantic Ocean than those of higher trophic groups such as zooplankton (Planque & Taylor 1998, Hays et al 2001, Lindley & Batten 2002) and fish (Fromentin et al 1998, Sims & Reid 2002, Lindley et al 2003. Since 1958, several studies have shown an increase in phytoplankton biomass in the North Atlantic (Barton et al 2003, Leterme et al 2005, the NE Atlantic (Reid et al 1987) and the North Sea (Reid 1978.…”

Occurrence of the basking sharkCetorhinus maximus(Gunnerus, 1765) (Lamniformes: Cetorhinidae) in the central-eastern Mediterranean Sea

“…In fact, C. maximus is known as a zooplankton feeder exhibiting typical behaviour in response to specific zooplankton densities and characteristics (Sims & Reid 2002;Sims et al 2003). Moreover, the higher availability of zooplankton in the Gulf of Taranto than in other parts of the Ionian Sea can explain the co-occurrence of pelagic and continental slope cetaceans such as Stenella coeruleoalba, Grampus griseus and Balaenoptera physalus (Di Matteo et al 2011).…”

“…The basking shark Cetorhinus maximus (Gunnerus, 1765) is a filter-feeding coastal pelagic species distributed worldwide in the Atlantic, Indian and Pacific Oceans (Compagno 1984), where it shows seasonal extensive horizontal and vertical migration in relation to the availability and displacement of its zooplanktonic food species (Sims & Quayle 1998;Francis & Duffy 2002;Sims & Reid 2002;Sims et al 2003).…”