Abstract
Harvest mortality typically truncates the harvested species' size structure, thereby reducing phenotypic complexity, which can lead to reduced population productivity, increased population variability, and selection on an array of life history traits that can further alter these demographic processes. Marine protected areas (MPAs) are a potential tool to protect older, larger individuals and therefore mitigate such ecological and evolutionary effects of harvest, depending on the degree of connectivity among areas. Such MPA protection relies on a shift in size-dependent mortality, the measurement of which can therefore serve as an early indicator of whether MPAs might achieve the desired longer-term ecological and evolutionary responses. We directly measured MPA effects on size-selective mortality and associated size structure using mark-recapture data on European lobster (Homarus gammarus) collected at three MPA-control area pairs in southern Norway during one decade (n=5,943). Mark-recapture modeling, accounting for variation in recapture probabilities, revealed (1) that annual mean survival was higher inside MPAs (0.592) vs. control areas (0.298) and (2) that significant negative relationships between survival and body size occurred at the control areas but not in the MPAs, where the effect of body size was predominantly positive. Additionally, we found (3) that mean and maximum body size increased over time inside MPAs but not in control areas. Overall, our results suggest that MPAs can rebuild phenotypic complexity (i.e., size structure) and provide protection from harvest selection.
Highlights
Individual vital rates determine the overall population productivity of harvested marine species, with older, larger individuals typically having disproportionately greater reproductive output than smaller and younger conspecifics (Birkeland and Dayton 2005, Barneche et al 2018)
By combining the analysis of body length data of European lobster with the analysis of a unique data set with thousands of individual encounters collected during a 12-yr mark–recapture study at replicated protected and harvested sites, we simultaneously examined changes in length-specific survival and size structure over time
We found that lobster survival and size complexity increased within protected areas whereas survival was lower and lobsters typically smaller in areas open to harvesting
Summary
Individual vital rates determine the overall population productivity of harvested marine species, with older, larger individuals typically having disproportionately greater reproductive output than smaller and younger conspecifics (Birkeland and Dayton 2005, Barneche et al 2018). Ecological Applications Vol 30, No 5 greater reproductive investment at earlier stages) that determine size-dependent vital rates, such that any resulting fisheries-induced evolution will further alter population productivity and variability (Hutchings and Fraser 2008). If such shifts are adaptive, a typical expectation is that they will increase the overall population growth rate compared to no adaptation for a given, ongoing level of intensive fishing. Fishing regimes that drive such evolutionary change can lead to population viability and biomass yield with lower robustness to uncertainty, stochasticity, and disturbances than management approaches that reduce fisheries-induced evolution (Ratner and Lande 2001, Baskett et al 2005, Audzijonyte and Kuparinen 2016 [but note an exception to this expectation in a model where growth is density dependent and growth speed trades off with body size; Kuparinen and Hutchings 2012])
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