Invasive species can have profound impacts on communities and it is increasingly recognized that such effects may be mediated by parasitism. The 'enemy release' hypothesis posits that invaders may be successful and have high impacts owing to escape from parasitism. Alternatively, we hypothesize that parasites may increase host feeding rates and hence parasitized invaders may have increased community impacts. Here, we investigate the influence of parasitism on the predatory impact of the invasive freshwater amphipod Gammarus pulex. Up to 70 per cent of individuals are infected with the acanthocephalan parasite Echinorhynchus truttae, but parasitized individuals were no different in body condition to those unparasitized. Parasitized individuals consumed significantly more prey (Asellus aquaticus; Isopoda) than did unparasitized individuals. Both parasitized and unparasitized individuals displayed Type-II functional responses (FRs), with the FR for parasitized individuals rising more steeply, with a higher asymptote, compared with unparasitized individuals. While the parasite reduced the fitness of individual females, we predict a minor effect on population recruitment because of low parasite prevalence in the peak reproductive period. The parasite thus has a large per capita effect on predatory rate but a low population fitness effect, and thus may enhance rather than reduce the impact of this invader.

Understanding the influence of spawning behaviour on the fine-scale distribution of Atlantic cod is essential to the design of effective conservation measures. Laboratory studies suggest that spawning activity occurs primarily at night, yet no field studies have evaluated the influence of diel period on the behaviour of individual wild spawning cod. Using an acoustic telemetry positioning system, the fine-scale movements of spawning cod were observed in situ as they returned to the same spawning location over consecutive seasons. The resulting data identify clear gender-based diel patterns in space use and aggregation behaviour among cod on a spawning ground. During the day, females remained aggregated in one small location that varied little within and between years. Males also aggregated during the day, but occupied a much larger adjacent area. At night, individual males sought out separate small territories while females generally remained near their daytime aggregation site, making periodic excursions into the surrounding area. These patterns were surprisingly stable over the 2 years of observation, indicating little interannual variability in spawning behaviour. This study provides an unprecedented examination of the natural spawning behaviour of Atlantic cod, and makes connections between earlier laboratory studies and field observations.

Migrations of springtime Alewife Alosa pseudoharengus and Blueback Herring A. aestivalis, collectively referred to as river herring, are monitored in many rivers along the Atlantic coast to estimate population sizes. While these estimates give an indication of annual differences in the number of returning adults, links to the subsequent timing and duration of spawning and freshwater juvenile productivity remain equivocal. In this study, we captured juvenile river herring at night in 20 coastal Massachusetts lakes using a purse seine and extracted otoliths to derive daily fish ages and back‐calculate spawn dates. Estimates of spawning dates were compared with fishway counts of migrating adults to assess differences in migration timing and the timing and duration of spawning. We observed a distinct delay between the beginning of the adult migration run and the start of spawning, ranging from 7 to 28 d across the 20 lakes. Spawning continued 13–48 d after adults stopped migrating into freshwater, further demonstrating a pronounced delay in spawning following migration. Across the study sites the duration of spawning (43–76 d) was longer but not related to the duration of migration (29–66 d). The extended spawning period is consistent with recent studies suggesting that Alewives are indeterminate spawners. The long duration in freshwater provides the opportunity for top‐down (i.e., predation on zooplankton) and bottom‐up (i.e., food for avian, fish, and other predators) effects, with implications for freshwater food webs and nutrient cycling. General patterns of spawn timing and duration can be incorporated into population models and used to estimate temporal changes in productivity associated with variable timing and density of spawning river herring in lakes.

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The Gulf of Maine fishery for northern shrimp (Pandalus borealis) has been a dynamic one, with landings varying greatly in response to resource and market conditions. A directed winter fishery developed in coastal waters in the late-1930s, which expanded to an offshore year round fishery in the late-1960s when annual landings peaked at about 13 000 tons in 1969. Landings subsequently declined to very low levels during the mid-1970s as recruitment failed and the stock collapsed, precipitating closure of the fishery in 1977. The resource recovered under restrictive management and was relatively stable at low to moderate levels of exploitation into the 1990s, with several strong year-classes recruiting to the fishery. In the mid-1990s, landings and fishing mortality increased sharply and abundance and recruitment have again declined. Environmental conditions have played an important role in affecting survival and abundance. Stock assessments have consistently shown that poor recruitment is more likely at low levels of spawning stock biomass owing to reduced total egg production. At a stock biomass level of 15 00020 000 tons, sustainable yields for the Gulf of Maine northern shrimp fishery appear to be 3 0004 000 tons per year under average environmental conditions.