Density-dependent growth in ‘catch-and-wait’ fisheries has implications for fisheries management and Marine Protected Areas

Abstract

Stock enhancement activities provide an opportunity to examine density-dependent suppression of population biomass which is a fundamental issue for resource management and design of no-take-zones. We document ‘catch-and-wait’ fisheries enhancement where all but the largest lobsters are thrown back, recapturing them later after they have grown to a larger size. The residency, rate of return, and potential negative density-dependent effects of this activity are described using a combination of tagging and v-notching and by relating spatial growth patterns to population density defined with Catch Per Unit Effort. The results successfully demonstrated the concept of catch-and-wait practices. However, a density-dependent suppression of growth (in body size) was observed in male lobsters. This demonstrates a mechanism to explain differences in lobster sizes previously observed across EU fishing grounds with different stock densities. This negative effect of density could also affect individual biomass production in marine reserve or no-take zones.Electronic supplementary materialThe online version of this article (10.1007/s13280-019-01158-1) contains supplementary material, which is available to authorized users.

Importance of suspended particulate organic matter in the diet of Nephrops norvegicus (Linnaeus, 1758)

“…during 'Spring long': 8 th March-29 th May (small and medium males compared to large males) and 'Summer long': 4 th May-25 th July (small males compared to large males). Males may suffer more competition for active food items than females 44 which may force smaller individuals to rely on particulate food sources. The same size-related difference, i.e.…”

“…As the present study shows, smaller and medium-sized males fed on significantly higher suspended food than larger ones, therefore the potential to suspension feed may be an important mechanism for avoiding aggressive encounters over food between males. This is potentially important because the growth (and hence biomass) of male individuals is strongly density-dependent at Clew Bay 44 (-densities at Clew Bay vary between 0 and 15 individuals per pot fished 45 ). Body size also varies across fishing grounds -smaller Nephrops are found at FUs with higher stock densities, most likely as a result of reduced growth potential due to intraspecific competition 5,44 .…”

“…This is potentially important because the growth (and hence biomass) of male individuals is strongly density-dependent at Clew Bay 44 (-densities at Clew Bay vary between 0 and 15 individuals per pot fished 45 ). Body size also varies across fishing grounds -smaller Nephrops are found at FUs with higher stock densities, most likely as a result of reduced growth potential due to intraspecific competition 5,44 . We suggest that feeding on POM is an important lifestyle adaptation in both males (counteracting competitive interactions) and females (counteracting burrow-dwelling) but that Nephrops diet is remarkably similar in the sexes.…”

Field-recorded data on habitat, density, growth and movement of Nephrops norvegicus

“…Mean body size and L max (theoretical size maximum) negatively correlate with density 1 . A mechanism has been shown for reduced body size at high densities in males, which has been linked to suppressed growth of inferior males in high density patches, although density-dependent mortality could not be detected in adult N. norvegicus 19 . Data to enable density calculations are provided in the present study.…”

Shift in the larval phenology of a marine ectotherm due to ocean warming with consequences for larval transport

“…The western Irish Sea population of N. norvegicus supports a highly productive fishery with yields of 5000–10,000 t annually and stock size has remained stable over the past 17 yr (Lundy et al 2019). It has high adult densities, although individuals are smaller in size relative to other grounds due to density‐dependent growth suppression (Johnson et al 2013; Merder et al 2020). Most larvae are present from April to June in the Irish Sea (Nichols et al 1987).…”