Demographic effects of full vs. partial protection from harvesting: Inference from an empirical before-after control-impact study on Atlantic cod

Abstract

In recent years, marine protected areas (MPAs) excluding all or some fishing activities have become widely applied as a tool to rebuild marine ecosystems and fisheries, but still, our knowledge of the effects of MPAs on the demography of harvested populations remains scarce, especially in cases where partly protected areas are still supporting some fishing activities. Here, we investigated the influence of partial protection - excluding fixed gears such as gillnets and traps but still allowing a hook and line fishery - on the demography of Atlantic cod Gadus morhua along the Norwegian Skagerrak coast. To do so, we applied novel multi-event models to a 9-year data set containing both live recaptures and dead recoveries of marked fish of different sizes collected before and after the implementation of an MPA, also including several unprotected sites. At the MPA site, the annual proportion of deaths due to fishing decreased from 0·59 before MPA implementation to 0·32 after MPA implementation. As a result, annual survival increased by 167% (from 0·09 to 0·24) for small (16-44 cm) and 83% (from 0·23 to 0·42) for big (45-97 cm) cod, respectively. Average survival was lower in unprotected areas, and annual fishing mortality was higher, representing almost 100% of the total mortality of large-sized cod in some years. Annual emigration rates (i.e. spillover from the MPA) increased in the last years of the study, but no differences in body growth were detected between sites. Closure of the fishery within the MPA would further increase the annual survival of smaller individuals by 100% (from 0·24 ± 0·07 to 0·48 ± 0·11) and that of larger individuals by 44% (from 0·42 ± 0·05 to 0·60 ± 0·09). Synthesis and applications. Our study reveals how current marine protected areas (MPAs) function and provides important insights for guiding a future adaptive management process. Given that harvested populations will not be able to sustain annual survival rates consistently below 30%, as found here; our results underscore the urgency of effective management measures such as no-take zones in areas where local populations are particularly reduced or in dire need of demographic rescue. Our study reveals how current marine protected areas (MPAs) function and provides important insights for guiding a future adaptive management process. Given that harvested populations will not be able to sustain annual survival rates consistently below 30%, as found here; our results underscore the urgency of effective management measures such as no-take zones in areas where local populations are particularly reduced or in dire need of demographic rescue.