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FEEDING OF MESOPELAGIC FISH

The Gadidae family is noted as existing within a range that encompasses both pelagic and benthic zones and is classified in various sources as pelagic, mesopelagic, or benthic. Local conditions probably contribute to variation within species, and particular species have tendencies towards different zones.

GADIDAE

Atlantic Cod • Gadus morhua

The Atlantic cod is the key fish predator in the southern Gulf of St. Lawrence (Majowski and Waiwood, 1980). Cod feed visually (Methven, 1999) on nektonic, epibenthic and shallow infaunal prey (Lilly, 1987). They are associated with cool temperate to subarctic waters on the continental shelf and inshore. Cod are described as voracious benthic feeders by Scott and Scott (1988), consuming a diverse diet, fish being a large diet component with the type varying by locality.

In St. Georges Bay, cod are among the most abundant fish by biomass and are found in deeper waters, mainly below 30 m. Cod enter the bay and spawn during May; eggs hatch and develop into larvae and juveniles. Their population peaks in midsummer, while the population of juveniles peaks in August. Some or all Atlantic cod leave the bay in winter (Kenchington, 1980). The adult population migrates first to deeper water in the autumn, followed by the juveniles (Clay, 1991).

A series of diet studies have been carried out in the southern Gulf of St. Lawrence, predominantly in the western portion, but no sampling data has been found within St. Georges Bay. Waiwood (1981) reports on historical data of stomach content by percent volume for over 20,000 cod stomachs taken from 1959 to 1973 and 1979 to 1980, distributed across the southern Gulf of St. Lawrence. The data, presented in detail by predator size and year, is presented in Appendix 4. Sampling sites were not evenly distributed, thus the data is somewhat biased toward the southwestern portion of the Gulf.

This data suggests a concentration of prey, and of predation by cod in the southwestern Gulf, major species consumed being capelin, flatfish, herring, gadoids and mackerel. Invertebrates consumed were predominantly krill, amphipods, mysids, polychaetes, and the decapods Crangon, Pandalidae, Pauridae, Hyas and Chionocetes. Krill, polychaetes, bivalves, snails, brittlestars, and amphipods were found in large quantities in cod diet in the southeastern Gulf, as well as flatfish and herring in the late fall.

Waiwood (1981) suggests that the occurrence of predation of particular species could be interpreted as representative of prey distribution. Thus, the data presented provides a general assessment of prey distribution from 1959 to 1973 throughout the southern Gulf. Despite the bias of sample sites to the western Gulf, there were clear differences in distribution of predation of various species. Capelin, mackerel and gadids were mostly restricted to the southwestern Gulf, but flatfish predation was more evenly distributed, and herring predation follows a pattern counterclockwise from the Laurentian Channel to the southern Gulf throughout the summer and autumn. Invertebrates were consumed throughout the southern Gulf, with a concentration in the west.

Much of the data pertaining to cod diet is presented by age group in percent weight, and particularly in earlier studies, in percentage occurrence. These factors hamper presentation of simple summary data, as well as the direct comparison of current to historical data. Table 22 presents diet information from the southwestern Gulf of St. Lawrence reported by Waiwood and Majowski (1984). Fish, particularly flatfish, crustaceans, chiefly decapods and krill, and also molluscs and annelids are all significant components of cod diet.

Table 22: Stomach contents of Atlantic cod from southwestern Gulf of St. Lawrence, adjusted to exclude unidentified categories, May to November, 1980 (Waiwood and Majowski, 1984).

Methven (1999) reviews diet studies of Atlantic cod and reports a general pattern reflected by the data presented in Table 22. Crustaceans and fish are always found to be the two most heavily consumed prey groups, with the former being fed upon more by smaller cod and the latter usually being consumed more by larger cod.

Kohler and Fitzgerald (1969) present data which illustrates similarities between Gulf of St. Lawrence (Magdalen Shallows), and Scotian Shelf cod diets. In both areas, there was a transition from consumption of mainly crustaceans by small cod to a fish diet with increased size. The different availabilities of prey between locations are reflected in a predominance of herring in Gulf diet, sand lance in Scotian Shelf diet (Kohler and Fitzgerald, 1969) and capelin in Newfoundland diet (Minet and Perodou, 1977). This flexibility in diet is common, as cod exhibits a preference for fish, particularly pelagic fish which are migratory and experience considerable variability in abundance between years (Methven, 1999).

Herring was found to be a significant contributor to the diet of cod in the 1950's and 1960's, (Kohler and Fitzgerald, 1969; Powles, 1958), however, Waiwood and Losier (1982) reported a reduction more recently in herring contribution to cod diet. They attributed earlier high consumption rates to an increase in herring availability due to increased vulnerability to predation because of disease. Their analysis revealed that the percentage of herring in cod diet was less than 1% when the herring abundance was low, however, at peak population, herring never contributed more than 14% to cod diet. Thus it appears that cod will reach a saturation level for prey species, given increasing abundances.

Capelin consumption by cod has been investigated in several regions of the Northwest Atlantic. In the northern Gulf of St. Lawrence, Minet and Perodou (1977) reported capelin as contributing greater than 75% of the diet of smaller cod ( 26 to 55 cm), but in cod of lengths 76 to 115 cm, the percent weight of capelin was reduced to less than 25%. Seasonal variation in diet was not observed to be substantial.

From 1977-78 area 4T (southern Gulf) diet data, Majowski and Waiwood (1980) calculated the maximal possible contribution of fish species to the cod diet as 14% capelin, 2.8% plaice, 0.2% herring and 4.3% for young cod.

Waiwood and Majowski (1984) found an increase in the consumption of fish, such as mackerel, cod, and flatfish with increase in predator size as well as increasing presence of larger crustaceans, such as Crangonidae, Pandalidae, and skates. Change in cod diet with size, was observed by Waiwood et al. (1980) as the prominence of pelagic prey such as krill, amphipods and capelin in small cod stomachs, and Powles (1958) reports an increase in benthic prey with size, noting that they begin to actually 'root-up' bottom fauna at 31-50 cm in length.

Powles (1965) discusses the potential for competition in the Magdalen Shallows between cod and American plaice, both of which being abundant fish in St. Georges Bay. Powles observed that more competition would occur among juveniles, as diet for both species is less specialized at this stage. Because plaice diet becomes more specialized with age, feeding chiefly on echinoderms and molluscs, the unspecialized diet of cod reduces competition with plaice, and competition for prey is limited to larger crustaceans.

Diet of cod from May to November of 1980 was examined by Waiwood and Majowski (1984) in the southwestern Gulf. Krill consumption was found to be highest in the spring, low in the summer and somewhat increased in the fall. In September and November, the contribution of fish is reduced while snow crab and molluscs increased in importance to cod diet. These authors suggest that significant changes in prey composition have corresponded to periods of optimum and sub-optimum feeding by cod, as the caloric content of prey species varies.

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Silver hake • Merluccius bilinearis

Silver hake are an abundant benthic species in the Northwest Atlantic, feeding nocturnally, and preferring warmer water than other gadids (Scott and Scott, 1988). They are new-comers to St. Georges Bay, not commonly known to be in the Gulf of St. Lawrence before the 1950's (Kenchington, 1980). Studies reviewed suggest that they are opportunistic feeders which feed primarily on gadids, including young hake, and a variety of pelagic species and crustaceans, primarily krill. Because of their abundance and versatility in feeding, they are thought to have a considerable potential role in regulating the Northwest Atlantic ecosystem (Edwards and Bowman, 1979, cited in Waldron, 1987).

Diet has been examined on the Scotian Shelf and Georges Bank as well as Newfoundland, although information was not found for silver hake within the Gulf of St. Lawrence. All studies find crustaceans and fish to be major preys items. Selected studies are presented in Table 23.

Waldron (1987) found that although crustaceans are numerically significant on the Scotian Shelf (44% frequency), fish contribute 48.5% to the diet of silver hake by weight. Waldron reports that young silver hake represented 25.4 of these percentage points. High cannibalism rates are believed to reflect a pattern of silver hake eating whatever is abundant. He also noted that silver hake feed primarily on fish that are pre-recruit to the fishery. Vinogradov (1972) did not find such a high incidence of cannibalism, but acknowledged its prevalence among silver hake. His studies on the Scotian Bank and the Georges Bank reported anchovy, herring, argentines, mackerel and sand lance as most common prey.

Bowman and Michaels (1984) found fish consumption to range from 76.4% to 78.4% of diet weight, herring and mackerel constituting the largest proportions of the diet. Vinogradov (1972) found invertebrate prey to consist mainly of the krill species M. norvegica and T. inermis; but also included shrimp and squid; fish being consumed to a lesser degree than apparent in other studies.

Table 23: Stomach contents of silver hake stomachs (percent weight) reported from Scotian Shelf, Southern New England, Georges Bank, and Western Nova Scotia: (1) 1981-1986, Waldron (1987); (2) Bowman and Michaels (1984); (3) 1969- 1972, Bowman et al. (1976).

Bowman and Michaels (1984) found crustaceans to contribute most to the silver hake diet. Among crustaceans, amphipods (Ampeliscidae, Parathemisto), decapods (Crangon septemspinosa and Dichelopandalus lepotcerus) and krill (exclusively M. norvegica) constituted the greatest biomasses. Fish consumed reflected Langton and Bowman's findings, and squid also contributed a significant proportion of prey. Bowman et al. (1976) provide comparable data from the same sites sampled by Bowman and Michaels (1984).

Young silver hake feed heavily on crustaceans (Waldron, 1987; Vinogradov, 1983), although adult silver hake continue to prey upon this group. Molluscs (squid) are also eaten as a transition to larger prey, and fish contributes a greater proportion of diet with predator age. The size of prey also increases with hake size. Bowman et al. (1987) found that young silver hake fed on amphipods (Caprillidae and Parathemisto), decapods (Crangonidae), mysids (Neomysis americana), and krill (M. norvegica) as well as small fish including silver hake and sand lance.

Seasonal variation in silver hake diet is attributed to fluctuation in abundance rather than varying predator preference. Waldron (1987) observed a shift in diet from predominantly fish in the spring to a mixture of fish, crustaceans and molluscs during the summer and a return to fish predominantly in the fall. As silver hake prey selection is reportedly influenced by seasonal species abundance, it may be expected that fish consumption in St. Georges Bay would increase over the summer, as migrant species populations increase.

Marked differences between male and female diets were found by Waldron (1987) as females were found to consume more fish, and males consumed more crustaceans.

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White Hake • Urophycis tenuis

White hake, or hake, as they are commonly called in the St. Georges Bay area, are notably a cold water fish that prefer water 5 - 11°C, but they are known to travel into warmer shoal areas during the summer (Scott and Scott, 1988). They arrive in the bay in June and appear to favour the mouth of the bay (Kenchington, 1980). This pelagic fish preys heavily upon crustaceans, as well as fish such as herring and gadids.

Published information on white hake diet in the Gulf of St. Lawrence was not found. Data from Bowman and Michaels (1984) for Northwest Atlantic sites are presented in Table 24. Fish comprised on average 88.5 % of the diet, unfortunately, a large proportion of the fish were not identifiable. Those identified included argentines, herring, mackerel, white, red and silver hake, redfish and winter flounder. Bowman et al. (1976) report very similar findings for two sites, but Georges Bank and Gulf of Maine diets were less reliant on fish and more upon crustaceans. Variation in diet between sites is not great, all diets contain a large fish component (43.0 - 88.9 %), gadids being significant in most sites, and sand lance in western Nova Scotia. Arthropods were differentially preyed upon, but in all cases decapod or krill were among the largest prey groups.

Typical diet items for juvenile white hake include calanoid copepods and cladocerons, with an average prey length of 0.74 mm (Coates et al., 1982). In the Bay of Fundy they are reported to feed on amphipods, nematodes, copepods and mysids (Imrie and Daborn, 1981; cited in Scott and Scott, 1988).

Bowman and Michaels (1984) found that more than one half of the young white hake diet in the Northwest Atlantic was of crustaceans; decapod shrimp and krill. Bowman et al. (1984) found that small white hake of 11 - 15 cm in length consumed polychaetes, amphipods and decapods and that 16 - 20 cm fish preyed upon decapod shrimp (D. leptocerus, Pandalus borealis), krill (M. norvegica) and mysids. Fish greater than 40 cm ate fish almost exclusively.

Limited information is available on seasonal variation in feeding. Bowman and Michaels (1984) found that white hake fed on various shrimp, mainly P. borealis and M. norvegica in the spring and fall.

Table 24: Percent weight of prey groups in stomachs of white hake in Northwest Atlantic sites; (1) 1973-1976, Bowman and Michaels (1984) and (2) 1969-1973, Bowman et al. (1976).

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Pollock • Pollachius virens

Among gadids, the pollock is the most inclined towards pelagic existence. Krill constitute the largest proportion of pollock diet in the Bay of Fundy, but fish such as herring, sand lance, silver hake, spotted lanternfishes and silversides contributed more to the diet of pollock in the Laurentian Channel and the Scotian Shelf (Scott and Scott, 1988). The population of pollock in the St. Georges Bay, at the time of an extensive sampling of fish in 1978 was estimated to contribute a negligeable biomass to the ecosystem. These fish enter the Bay in the spring, but return to the Gulf of Maine to spawn in the winter (Kenchington, 1980).

Pollock diet in the northwest Atlantic reported by Bowman et al. (1976) are presented in Table 25. Feeding range is quite narrow, as only 2 groups of crustaceans and fish constitute large prey components. Crustaceans range in composition from 51.1 % - 70.7 %, particularly krill and caridean shrimp, the latter in larger proportions only in the Gulf of Maine. Fish range from 23.6 % to 46.9 % in terms of contribution to the diet, but species are largely unidentified.

Table 25: Percent weight of prey groups from stomachs of pollock from Northwest Atlantic sites, 1969-1973, Bowman et al. (1976).

Bowman and Michaels (1984) found consistently higher consumption of fish and lower consumption of crustaceans than an earlier study of the diets of these sites by Bowman et al. (1976). Bowman and Michaels (1984) reported that fish comprised 47.2% of the diet and shrimp contributed 38.8% to the overall diet of pollock in their study of Gulf of Maine and western Nova Scotia. Squid was also a component of the diet contributing 7.7%, when available. Maurer and Bowman (1975) found the importance of these former two groups reversed, contributing 31.9% and 65.5% weight to the pollock diet, respectively.

Bowman and Michaels (1984) found that the krill species, M. norvegica and T. inermis, and the decapod shrimps, Pasiphaea multidentata, P. borealis and D. leptocerus constituted 60% of the diet of fish less than 65 cm. Fish was also significant by weight in pollock diet, but constitute a larger proportion of diet for pollock over 65 cm in length. Common fish prey were identified as silver hake, pollock, headlight lanternfishes, mackerel and redfish. No obvious seasonal changes were observed, although fluctuations among squid, juvenile pollock and silver hake compositions of diet occurred. This was attributed to changes in prey abundance.

Young pollock are believed to feed in shallow waters on small crustaceans, particularly amphipods (Scott and Scott, 1988). Bowman et al. (1987), however, found that M. norvegica contributed greater than 90 % by weight to the diet of juvenile pollock.

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Fourbeard rockling • Enchelyopus cimbrius

Fourbeard rocklings exhibit a preference for soft muddy bottom. Little is known of their feeding, but they are believed to feed on crustaceans, polychaetes and molluscs (Scott and Scott, 1988). Although they are common to the Southern Gulf, they are usually found at depths greater than 50 m, so may not be prevalent in St. Georges Bay (Kenchington, 1980). Tyler (1972) identified the krill species, M. norvegica; the isopod species Unciola leucopis and Maera loveni and the polychaete Nephtys incisa as the major prey of the fourbeard rockling in the Passamaquoddy Bay, New Brunswick.

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