Effects of population density and body size on disease ecology of the European lobster in a temperate marine conservation zone

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August 30, 2016 - 9:40am
Special thanks to OpenChannels member Charlotte Eve Davies for submitting this content!
Type: Journal Article
Year of publication: 2014
Date published: 12/2014
Authors: Charlotte Davies, Andrew Johnson, Emma Wootton, Spencer Greenwood, Fraser Clark, Claire Vogan, Andrew Rowley
Journal title: ICES Journal of Marine Science
ISSN: 1095-9289

Marine conservation zones (MCZs) are a form of spatial marine management, increasingly popular since the move towards ecosystem-based fisheries management. Implementation, however, is somewhat contentious and as a result of their short history, their effects are still widely unknown and understudied. Here, we investigate the population and health of the European lobster (Homarus gammarus) in the Lundy Island Marine Conservation Zone, Bristol Channel, UK. Using the fished refuge zone (RZ) as a control area, catch per unit effort was calculated for both the no-take zone (NTZ) and RZ and binomial logistic regression models were used to examine the effects of site, sex, landing size, and loss of chelae on the probability of shell disease and injury presence in individuals. Lobsters were also tested for the causative agent of gaffkaemia, Aerococcus viridans var. homari, and white spot syndrome virus (WSSV). The analysis revealed a higher lobster density and larger lobsters in the NTZ compared with the RZ. Shell disease was present in 24% of lobsters and the probability of shell disease occurrence increased notably for individuals over the minimum landing size (MLS) of 90 mm carapace length. Shell disease was also more prevalent in lobsters displaying injury, and in males. Injury was present in 33% of lobsters sampled and prevalence was higher in lobsters in the NTZ compared with the RZ, and in lobsters >MLS. Aerococcus viridans var. homari was detected in <1% of individuals, but WSSV was absent from all sampled lobsters. Overall, the study demonstrates both positive and potentially negative effects of NTZs, methods for effective non-lethal sampling of disease agents, and highlights the need for more comprehensive, long-term monitoring within highly protected MCZs, both before and after implementation.

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Why the negative spin on a consequence which anybody with even a basic understanding of population ecology would accept and even welcome as inevitable?

Density dependence is a well recognised central tenet of population ecology, i.e. as the density of a population is restored back to unexploited levels, a number of 'natural' trends will increase, such as increased prevalence of disease amongst more crowded populations and older 'senile' individuals (as natural age structure is restored), along with increased competition for space, sexual partners, food, etc., leading to increased fighting related injuries. Per capita production will also decrease due to competition for food, cannibalism, etc. This is naturally what happens when you stop thinning a population through harvesting. It certainly is not hidden or unexpected, not is it a threat to a successful marine conservation story. It is simply what should be expected to happen when a population is restored back to natural levels.

The negative spin related to findings such as this are eagerly seized upon by the fishing industry and some fisheries scientists [1]  as arguments against no-take MPAs, when they should be more positively presented as a natural consequence of the recovery of populations back to unexploited densities and age structures, along with related spill-over and export benefits. I have been presented with a partial understanding of such findings by several fishermen as yet another reason why no-take MPAs are a bad idea. It would be better if these findings could be presented in a more balanced way that recognises that they are merely a representation of basic population ecology associated with the recovery of marine population back to natural unexploited levels, rather than as the negative 'rarely broadcast'  threatening and hidden side effects of no-take MPAsWhy the negative spin on a consequence which anybody with even a basic understanding of population ecology would accept and even welcome as inevitable?

[1] Jones P.J.S. (2007) Point of View - Arguments for conventional fisheries management and against no-take marine protected areas: only half of the story? Reviews in Fish Biology and Fisheries 17(1), 31-43. doi:10.1007/s11160-006-9016-8 - Copy of paper. See also pp.46-55 on Divergent views and the quest for common ground amongst fisheries scientists and marine ecologists in Jones P.J.S. (2014)Governing Marine Protected Areas: resilience through diversity (see previous MPA News interview on this book)

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