(1) Female wading birds arrive at the head of the Bay of Fundy in early to mid-July carrying a moderate parasite load. Over the three weeks they spend there, the parasite load drops as they feed on Corophium, doubling their weight before starting the long (3,000 miles), non-stop haul to South America where they overwinter. As they are leaving the mudflats, the males and juveniles are arriving also carrying moderate parasite loads. However, during the time the females were there, the Corophium (and snails and polychaetes) were becoming infected with various parasite larval stages (from the parasites the females brought in). These infective larvae are then available to the males and juveniles so that by the time they start their southern migration, they are carrying relatively heavy parasite loads with some mortality occurring en route. (The proportion of mature females arriving on the overwintering grounds is higher than that of mature males and juveniles)

The above descriptive scenario is being examined on a quantitative basis this summer. A UNB Honours student (Nicole Bourgeois) is examining the prevalence and intensity of parasites in Corophium, snails and polychaetes establishing the baseline data during June, and documenting quantitative changes in parasite burdens over the period that the mature female birds, and then the mature males and juveniles, during July and August. Helping with this project are two colleagues of mine, Dr A.S.Didyk and Dr D.G.Crowe.

(2) It is well-established in the literature that factors causing stress can be accumulated by organisms to the point that they cause death. The early work of John Sprague (then at DFO, St Andrews) established the concept of lethal

units w.r.t. heavy metals. For example 48 ppb copper would kill juvenile Atlantic salmon whereas it took 600 ppb of zinc. These constituted lethal units for these metals when present in the absence of other toxic metals. However, half of the lethal unit for copper (24 ppb) plus half of the lethal unit for zinc (300 ppb) would make up a complete lethal unit that would kill fish. Taken further, this means that 10% of the lethal units of 10 toxic metals could make up a lethal unit capable of causing mortality. There are a number of instances where it has been shown that combinations of certain metals can decrease the individual toxicities and other instances where certain combinations can exacerbate the individual toxicities. Work that I have been involved with in recent years looks at parasitism as a stress factor which, in combination with other stress factors such as heavy metals, is cumulative and can lead to death of the host organism.

As part of the Collaborative Mercury Research Network (COMERN) Centre of Excellence, a group of us have been looking at mercury levels in various organisms, at different trophic levels in the food chain, in the Bay of Fundy

coastal ecosystem (the New Brunswick side of the Bay). My interest, specifically, is to relate these levels to parasite loads and to mercury levels within the parasites themselves (as the "top" predator). Where metal levels are relatively high, we find no fish with large numbers of parasites but where metal levels are low, there is always a small proportion of fish with heavy parasite loads. This suggests that in the presence of significant metal toxicity, the fish cannot survive the extra stress caused by large numbers of parasites and such heavily parasitised fish die. In some cases, the

mercury levels in the parasites are much higher than in the fish they parasitise which could be interpreted as being beneficial to the fish by removing metals that might otherwise have been taken up by the fish. Helping me with this work is a graduate student (Chris Blanar) and two undergraduate students (Jesse Bourque and Georgina Cox).