Grass pickerel (Esox americanus vermiculatus) COSEWIC assessment and status report: chapter 8

Biology

General

This is a warmwater fish with an affinity for vegetation, capable of surviving in small bodies of water without flow in summer and covered by ice in winter. At different times of the year, it occupies different sections of the habitat described. Its main spawning period is in the spring when water is plentiful and new growth of vegetation has started. It shares these habitats with a variety of other warmwater species such as suckers, catfishes, sunfishes, pikes, and minnows. It is a sight predator, with young fish feeding on a wide variety of organisms starting with small invertebrates, shifting largely to fishes and crayfishes as they grow. Adult grass pickerel are cannibalistic at certain times (Crossman 1962a). Females grow to larger sizes than males. Maximum scale ages vary over the distribution from 4 yr in Wisconsin, Ohio and Oklahoma (Kleinert and Mraz 1966, Trautman 1981 and Ming 1968) to 7 yr in Ontario (Crossman 1962a). Age from cleithra would be more dependable. Maximum recorded size in Canada (Severn River) is 328 mm TL (total length) and 204 g, and in the United States (Ohio) 381 mm TL and 397 g (Scott and Crossman 1973).

Reproduction

Very few details of the actual spawning activity of this fish have been reported. It has been considered to occur in, or at, the edge of pads of vegetation. Toner (1943) reported that spawning pickerel stay closer to shore than do northern pike. He noted that a large female grass pickerel is associated with a number of smaller males as in other esocids. Adults reach sexual maturity in the first year of life in Wisconsin, but apparently only in the second year in Ontario. No nest is built and neither eggs nor young are provided any parental care. The eggs are demersal, slightly adhesive, and adhere to vegetation. Neither reproductive migration nor homing are known, but McNamara (1937) suggested male grass pickerel were the first fishes to move upstream after the ice has disappeared, the females followed later, and spawning takes place in temporary floodplain marshes. Spawning takes place in late February to March in Oklahoma (Ming 1968), April in Pennsylvania and Wisconsin (Buss 1962, Kleinert and Mraz 1966), and late March to early May in Ontario (Crossman 1962a). In Ontario, spawning takes place in water temperatures approximately 8-12°C, eggs hatch in 11-15 days at temperatures of 7.8-8.9°C, and the time period between spawning to initiation of feeding by young involves 2-5 weeks depending on water temperature. In Pennsylvania, it was recorded that the grass pickerel runs with the northern pike to a quiet vegetated area to spawn (Buss 1963).

Details on developing newly hatched individuals from Ontario were provided by Leslie and Gorrie (1985) and for the Ohio River system by Yeager (1990).

The grass pickerel, apparently unlike its sister subspecies, has long been known to spawn in the late summer to winter. Evidence for this includes: the presence of individuals late in the year (October-December) that were similar in size to those in June (Lagler and Hubbs 1943, Crossman 1962a and Ming 1968); presence of two age classes of individuals less than 78 mm TL in October: 2 individuals, 33 and 42 mm, with 6 and 12 circuli, and 4 larger individuals with 26-39 circuli (Crossman 1962a); and presence of ripe females in August-November (Crossman 1962a, Kleinhart and Mraz 1966). Kleinhart and Mraz (1966) suggested that grass pickerel spawn more than once per year due to the occurrence of eggs of varying ripeness and size in the same individual.

Physiology

This subspecies is adapted to hightemperatures. Final preferred temperature (experimentally determined) is 25.6°C, and maximum water temperature in some successful habitats was 28.9°C. Tolerance level for dissolved oxygen was recorded at 0.3-0.4 ppm (Cooper and Washburn 1949). Although the esocid fishes are generally considered to be primary freshwater species, there is a varying amount of saltwater tolerance. The highest salinity known for Esox americanus americanus was 14 ppt (Schwartz et al. 1982).

Movements/dispersal

Movements associated with spawning, particularly where habitats are ice covered in winter, are given under Reproduction, above. Grass pickerel, when undisturbed, are often observed near shore, or at the outer edge of patches of vegetation, oriented with the head toward the shore or vegetation. There is vertical distribution with the younger individuals near the surface and the adults in deeper water, if it exists. Movements within streams do not appear to be extensive, but they must move in regard to lowering water levels and become concentrated in deeper, even isolated pools.

Nutrition and interspecific interactions

Considerable detail on size of pickerel and number and volume of food items consumed were presented by Crossman (1962a), Kleinert and Mraz (1966), and Ming (1968). The food of individuals less than 50 mm length in Jones Creek consisted of Cladocera, Amphipoda, Ostracoda, Odonata, and less frequently Diptera, Plecoptera, Hemiptera and Isopoda (Crossman 1962a). In Oklahoma, some pickerel in this size range had eaten fishes and tadpoles (Ming 1968). In Ontario, grass pickerel 50-100 mm in length started feeding on fishes, but the diet was mainly Trichoptera, Odonata, and crayfishes (Crossman 1962a). In Ontario, frogs and tadpoles were infrequently eaten although they were very abundant. The diet of individuals in larger length groups shifted gradually such that fishes and crayfishes dominated, but nymphs of aquatic insects still appeared. This pattern appears general since it agrees with that recorded for Oklahoma (Ming 1968) and Tennessee (Rice 1942), although grass pickerel in Oklahoma ate a number of other aquatic vertebrates.

Cannibalism occurred infrequently, and there appears to be no evidence that this fish gorges on fishes. In Ontario, rarely were there more than two fishes in the stomach of a grass pickerel.

Interactions with other species of fishes were limited to predation and food. In Jones Creek, there were 22 other species of fishes, but the grass pickerel preyed on only nine. The central mudminnow, Umbra limi, and the golden shiner, Notemigonus crysoleucas were dominant prey items. Golden shiners were preyed upon in relation to their relative abundance rather than selection (Crossman 1962a), but Crossman (1962b) suggested that the grass pickerel selected for the central mudminnow. Ming (1968) noted that, in Oklahoma, of 76 species of fishes captured, only 44 of them could be said to be "closely associated" with the grass pickerel. Those species were in the following families: Lepisosteidae, Amiidae, Clupeidae, Cyprinidae, Catostomidae, Ictaluridae, Anguillidae, Centrarchidae, Percidae, Sciaenidae and Atherinidae.

Becker (1983) indicated that, in Wisconsin, grass pickerel are eaten by catfishes (Ictaluridae), sunfishes (Centrarchidae), yellow perch (Perca flavescens), and grass pickerel. Extensive accounts of diets of common piscivorous birds--osprey (Pandion haliaetus), common loon (Gavia immer), double crested cormorant (Phalacrocorax auritus), common merganser (Mergus merganser), belted kingfisher (Ceryle alcyon), and great blue heron (Ardea herodias)--in the Birds of North America Series (Poole and Gill, eds., 1992-2002) were checked. None of the literature surveyed indicated that the grass pickerel was eaten by any of these fish-eating birds that are common in the same habitats. An assumption prevails that grass pickerel may be detrimental to northern pike, and Kleinert and Mraz (1966) suggested that management efforts should be made that would prevent the spread of the pickerel.

Grass pickerel are known in nature to hybridize with redfin pickerel, chain pickerel, and northern pike (Serns and McKnight 1977, Schwartz 1962, Schwartz 1981). Artificial hybrids between muskellunge and grass pickerel lived at least 18 months (Tenant and Billy 1963, Crossman and Buss 1965).

Jones Creek individuals were parasitized by 11 organisms, mostly trematodes, in virtually all internal organs. Only three protozoans appeared dense enough to affect the health of grass pickerel (Crossman 1962a, see also Ming 1968).

Behaviour and adaptability

Other than the apparent habit of individuals orienting with the head toward the shore around the edge of a pond, the behaviour of this fish is not markedly different from that well documented for the better-known esocid species. The nature of the habitats occupied in rather significant numbers suggests this fish is highly adaptable. The ability of the grass pickerel to become established in areas outside its native range as a result of accidental or authorized introductions also suggests adaptability.

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