BackgroundEcologists, fisheries scientists, and coastal managers have all called for an ecosystem approach to fisheries management, yet many species such as the American lobster (Homarus americanus) are still largely managed individually. One hypothesis that has yet to be tested suggests that human augmentation of lobster diets via the use of Atlantic herring (Clupea harengus) as bait may contribute to recent increases in lobster landings. Currently 70% of Atlantic herring landings in the Gulf of Maine are used as bait to catch lobsters in traps throughout coastal New England.Methodology/Principal FindingsWe examined the effects of this herring bait on the diet composition and growth rate of lobsters at heavily baited vs. seasonally closed (i.e., bait free) sites in coastal Maine. Our results suggest that human use of herring bait may be subsidizing juvenile lobster diets, thereby enhancing lobster growth and the overall economic value and yield of one of the most valuable fisheries in the U.S.Conclusions/SignificanceOur study illustrates that shifting to an ecosystem approach to fisheries management should require consideration of cross-fishery interactions.

During the winter and spring of 2002 and 2003, we used time-integrated fertilization assays to monitor sperm availability in three populations of the green sea urchin Strongylocentrotus droebachiensis in Maine: a naturally occurring population of .40,000 urchins and two smaller groups (,1,000) of transplanted urchins isolated from other aggregations. Episodes of sperm release coincided in two populations 10 km apart, suggesting that urchins were responding to a widespread environmental signal. We observed significant lunar periodicity in sperm release events for both of these populations. However, extensive spawning as shown by fertilization rates near 100% and a dramatic drop in gonad mass only occurred in the large natural population around the onset of thermal stratification and the spring phytoplankton bloom. By contrast, in the two small populations we observed low fertilization rates and little or no change in gonad mass. We speculate that a subset of males in these populations responded to a common external spawning signal, but that mass spawning is more likely to occur in large, dense populations where sperm concentrations reach high enough levels to trigger spawning in less responsive urchins.

Lobster (Homarus americanus) landings in Maine, United States have increased steadily over the past two decades to levels that are consistently more than triple the 40-year (1950 to 1990) annual average of c. 9000 MT. Meanwhile, the use of herring (Clupea harengus) as bait has increased fourfold since the 1970s, and is currently subsidising lobster productivity in mid-coast Maine by augmenting the diet and growth of large juvenile and young adult lobsters before they recruit to the fishery. We investigated whether herring bait in the eastern portions of the Gulf of Maine is also subsidising these lobster populations. Harbour consumed markedly more bivalves than those at Cutler. Furthermore, quantification of trap densities and the number of licensed fishers in midcoast versus eastern Maine determined that fishing effort was greatly reduced in eastern Maine. Thus, our results indicate thatherring bait subsidies are not important in eastern portions of the Gulf of Maine, but the availability of natural prey seemingly limits the growth of lobsters in eastern Maine. Moreover, our study suggests that bottom-up forcing (i.e., food limitation) can have important consequences for lobster population dynamics and the productivity of lobster fisheries.

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Previous studies have documented female American lobster, Homarus americanus, carrying “abnormal clutches”, i.e. with eggs covering less than half of their abdomen. From 2011 to 2014, we worked alongside harvesters to quantify spatio-temporal patterns and reproductive costs of abnormal clutches among 138 738 egg-bearing female lobsters sampled from 193 homeports spread across eastern Canada. Females with abnormal clutches were ubiquitous, being found in 90% of homeports. Their incidence was, however, relatively low, averaging 6% across sampling times and locations. The incidence decreased between spawning and 7–9 months after spawning, potentially due to cases of complete brood failure caused by sperm limitation, and it then increased toward the end of the brooding period, potentially due to repeated catch and release of ovigerous females during the fishery. In most regions, small females were more likely to carry abnormal clutches than larger females. We estimated population-level egg loss between late oogenesis and hatching of embryos at 47–51%, with approximately half being associated with normal clutches and half with abnormal clutches and complete brood failure.