Long-term variability in bivalve recruitment, mortality, and growth and their contribution to fluctuations in food stocks of shellfish-eating birds

Abstract

Bivalve biomass in coastal marine areas tends to fluctuate heavily from year to year, but the relative impact of underlying processes (such as recruitment, mortality, and growth) on this variability is poorly known. The present study deals with 3 bivalve species that live on tidal flats and are readily exploitable as food for shellfish-eating birds, which have suffered in recent years from short food supply in the Wadden Sea. Long-term (~40 yr) monitoring of bivalve populations on tidal flats in the western Wadden Sea has revealed high year-to-year variability in numerical densities, whereas variability in growth rates and resulting mean individual weights-at-age was relatively low. High biomass values were found in particular when strong cohorts had arisen from successful recruitment in preceding years, whereas failing recruitment for some years in succession led to low biomass of bivalve stocks. Occasionally, temporarily enhanced mortality rates resulted in temporarily strongly reduced biomass values. Only in Macoma balthica were biomass values (at similar foregoing recruitment and mortality levels) lower in years with below-average individual weights than after years with above-average ones. In all 3 species, processes that govern numbers rather than individual weights were most decisive for biomass variability. Recruitment and mortality rates in only 3 species together explained a significant proportion of the variability of the total (multi-species) bird-accessible bivalve biomass. The low bird-food supply in recent years in the Wadden Sea appears to be due to a shift in the major structuring factors since the mid-1990s: as a consequence of the absence of cold winters in the area after 1997, annual recruitment in some major bivalve species has consistently failed and was not sufficiently compensated by better survival. A recent period of consistently enhanced mortality rates in M. balthica even aggravated the situation.

The impact of estuarine salinity changes on the bivalves Scrobicularia plana and Cerastoderma edule, illustrated by behavioral and mortality responses on a laboratory assay

“…The Mondego estuary (Portugal) is economically important to local human populations, which explore valuable biological resources, such as the bivalves Scrobicularia plana (da Costa, 1778) and Cerastoderma edule (L.) (Verdelhos et al, 2005(Verdelhos et al, , 2014Crespo et al, 2010). Bivalves are a link between primary producers and epibenthic consumers, with a key role on the community energy fluxes, and contribute to water column purification, filtering organic matter (Dumbauld et al, 2009;Beukema et al, 2010;Santos et al, 2011). the organism.…”

Aggregated prey and predation rates: Juvenile shore crabs (Carcinus maenas) foraging on post-larval cockles (Cerastoderma edule)

“…The key period for the successful recruitment and establishment of juvenile bivalves is during the early post-larval benthic stage (0.35-5 mm shell length) (Armonies, 1996;Hiddink and Wolff, 2002). The monitoring and management of C. edule are made more difficult by not only its highly variable inter-annual recruitment into fisheries (defined as ≥20 mm shell length in UK fisheries) (Beukema et al, 2010), but also its highly patchy distribution within populations. Post-larval C. edule settle onto north eastern Atlantic shores during early summer in high densities and show patchiness in their distribution within the first year following settlement (Armonies, 1996;Bouma et al, 2001;Seed and Brown, 1977).…”

Behavioral and mortality responses of the bivalves Scrobicularia plana and Cerastoderma edule to temperature, as indicator of climate change's potential impacts

“…Bivalves are one of the most important biological groups on estuaries, with an essential role on the community energy flow, contributing to water column purification and as an important food resource (Dumbauld et al, 2009;Cardoso et al, 2008;Beukema et al, 2010;Crespo et al, 2010;Santos et al, 2011;Parada et al, 2012;Verdelhos et al, 2014). Scrobicularia plana is a deposit-feeder with a wide geographic distribution along the NE Atlantic coast and Mediterranean Sea (Latitude: 60 • N to 14 • N), usually inhabiting muddy sediments to a depth of 25 cm (Santos et al, 2011;Verdelhos et al, 2011 and references therein).…”