Interpreting large-scale experiments on effects of trawling on benthic fauna: an empirical test of the potential effects of spatial confounding in experiments without replicated control and trawled areas

Disturbances due to trawling and dredging is a serious threat to assemblages of benthic marine animals. We tested hypotheses about effects of trawling on benthic assemblages in a manipulative field experiment, using gear and intensities relevant to future management of trawling in a Swedish fjord. Three trawled and three control sites were sampled at several times before and after trawling was initiated. This paper describes how conclusions about effects of trawling might differ between experiments involving replicate sites and experiments using only one trawled and one control site, as in several recent studies. Analyses of selected taxa showed that abundances of many species changed differently among control sites. Differences in temporal change between pairs of single trawled and control sites were also frequent. Neither the quantitative nor the qualitative nature of differences between treatments could, however, be coherently interpreted among the different combinations of trawled and control sites. This is consistent with results obtained from analyses using all sites, which showed no consistent effects of trawling on any of these taxa. These results provide empirical evidence that spatial confounding may cause serious problems to formal interpretation of experiments, which use only one control and one trawled area. Such potential problems can best be solved by ensuring that the study incorporates more than one control site.     

Modelling harvesting strategies for the lobster fishery in northern Europe: the importance of protecting egg-bearing females

European lobster populations in Norway and Sweden are severely reduced as a result of intense harvesting over a long time. Various alternative management options have been proposed or endorsed to both facilitate recovery and increase yield. Accordingly, Minimum Landing Size (MLS) regulations are widely used for the European lobster. We developed an individual-based population model which integrates biological knowledge about lobsters’ population dynamics to explore how available harvesting strategies and management options influence abundance and yield. The model reproduced basic features of a real lobster population in Sweden. Even for a relatively large MLS high fishing effort may still be detrimental to the long term production of the stock, while increasing the MLS further prevents this recruitment overfishing. A moratorium on berried females, in combination with the MLS appears to stabilize population fluctuations and yield, leading to higher yield for all MLS's considered. The female moratorium harvesting strategy also performed better than a maximum size limit. Yield per recruit calculations gave similar quantitative results, and also shows that a larger MLS reduce the risk of growth overfishing. A smaller MLS enables the harvest of many individuals but is very sensitive to increase in effort which easily promotes overfishing.     

Multi-Annual Fluctuations in Reconstructed Historical Time-Series of a European Lobster (Homarus gammarus) Population Disappear at Increased Exploitation Levels

Through the history of ecology, fluctuations of populations have been a dominating topic, and endogenous causes of fluctuations and oscillations have been recognized and studied for more than 80 years. Here we analyzed an historical dataset, covering more than 130 years, of European lobster (Homarus gammarus) catches. The data shows periodic fluctuations, which are first dampened and then disappear over time. The disappearance of the periodicity coincided with a substantial increase in fishing effort and the oscillations have not reappeared in the time series. The shifting baseline syndrome has changed our perception of not only the status of the stock, but also the regulating pressures. We describe the transition of a naturally regulated lobster population into a heavily exploited fisheries controlled stock. This is shown by the incorporation of environmental and endogenous processes in generalized additive models, autocorrelation functions and periodicity analyses of time-series.