Patterns of extirpation. II. The role of connectivity in the decline and recovery of elasmobranch populations in the German Bight as inferred from survey data

Abstract

Abundance trends and distribution patterns from 1902 to 1932 for 7 elasmobranch species in the German Bight (eastern central North Sea) were analyzed and compared to survey trends for 1991 to 2009. Abundances of thornback ray Raja clavata, common skate Dipturus batis, smoothhound Mustelus spp., tope shark Galeorhinus galeus, and spiny dogfish Squalus acanthias declined and common skate was extirpated, while abundances of starry ray Amblyraja radiata and lesser spotted dogfish Scyliorhinus canicula increased. A meta-population approach was developed to analyze connectivity in terms of the surrogate parameter Ĉ representing donor population N j , separation distance D i,j , and migration range parameter α. As populations declined, connectivity between sub-populations was lost before resident populations finally collapsed. The loss of connectivity was caused by both a loss of sub-populations and a subsequent increase in the distance separating the remaining sub-populations. For lesser spotted dogfish, an increase in the donor population fostered the establishment of a local sub-population after 2000. For starry rays, an apparent increase in N j due to a concentration of the donor population near the study area increased connectivity with the local sub-population. The interpretation of local abundances in terms of meta-population dynamics demonstrates the importance of seasonally resolved survey information for conservation planning.

Distribution of skates and sharks in the North Sea: 112 years of change

“…The interpretation of these data, however, requires a degree of caution, due to possible changes in sampling methods over time – issues that are, unfortunately, often associated with long‐term, historical data sets that predate full standardisation of surveys and documentation of their gears (Ellis et al ., ; Bartolino et al ., ; Fock et al ., ). Despite these issues, the present study can help inform on both the long‐term population dynamics of North Sea elasmobranchs and the assemblage at a time of low fishing impacts (Walker & Heessen, ; Rogers & Ellis, ).…”

“…S4.2). Over this period, beam trawl pressure has been higher in the eastern than western North Sea (Engelhard et al ., ) and has likely impacted not only adult skates but also egg‐laying and nursery grounds in coastal zones and outer estuaries (Frid et al ., ; Ellis et al ., ; Kinney & Simpfendorfer, ; Fock et al ., ). Our results show that, until the 1960s, the overall skate distribution coincided with that of thornback ray, thought to be the historically dominant species.…”

Identifying core areas for mobile species in space and time: A case study of the demersal fish community in the North Sea

“…Given the huge amount of surface area designated to offshore wind farms, marine spatial planning could support the conservation of demersal fish by excluding fishing in wind farms, which may provide important habitats for sensitive and threatened fish species e.g. elasmobranchs (Fock et al, 2014) or large gadoids (Reubens et al, 2014). It should be noted, however that offshore wind parks might not cover all relevant consistent core areas as wind parks cluster in the southern North Sea.…”