Ameiurus nebulosus (brown bullhead)

A. nebulosus is a fish of the Ictaluridae family, commonly known as a brown bullhead. It has been introduced outside of its native range in North America to other areas of North America, Europe, Asia and Pacific islands (i.e. New Zealand, Hawaii). It was likely spread primarily for recreational angling opportunities. Its spread has been undoubtedly facilitated by its ability to survive low oxygen concentrations for prolonged periods (Scott and Crossman, 1973). Its establishment, once introduced, was likely assisted by its generalist, omnivore diet with feeding aided, even in turbid waters, by its chin barbels (Scott and Crossman, 1973). This diet results in predation on a wide variety of native invertebrates, small vertebrates and fish eggs. Conversely, its stout shape and strong dorsal and pectoral fin spines would minimize predation by native predators. Its parental care of eggs and young would also reduce mortality in the young (Scott and Crossman, 1973). The species is recognized within the Global Invasive Species Database (ISSG, 2009).

Within its native range, two subspecies have been identified, Ameiurus nebulous nebulosus, north of a line between Viriginia and North Dakota, and Ameiurus nebulosus marmoratus to the south (Hubbs and Lagler, 1957). Although initially of the Ictalurus genus, Robins et al. (1980) concluded that Ameiurus was valid and was subsequently recommended by Bailey and Robins (1988).

A. nebulosus was described thoroughly by Scott and Crossman (1973) using specimens sampled from Ontario and New Brunswick, Canada: “The species is a moderate-sized catfish with adult total lengths between 203-356 mm. Its greatest body depth occurs at the origin of the dorsal fin, typically 17.7-26.3% of total length. The caudal peduncle (tail fin) is moderately deep (depth 8.1-9.9% of total body length) with a straight or slightly rounded posterior edge. The head is massive and somewhat lengthy, usually 22.6-26.3% of total body length, and quite wide throughout. Its eyes are small, with diameter 10.0-18.7% of head length, round, and protruding. Lips are fleshy, but not prominent, with one pair of long flattened maxillary barbels, which are the longest of four pairs of barbels. Its gill rakers are moderately long and pointed. Fins are as follows: 1 dorsal, ahead of midpoint of body, which is soft-rayed but with a strong leading spine. Pectoral fins are high, broad, rounded, with anterior heavy spine with numerous barbs. Spines in dorsal and pectoral fins can be locked in erect position, presumably as a predation deterrence strategy. The adipose fin is small; pelvic fins inconspicuous; and, anal fin long and slender. The skin lacks scales throughout but is complete with taste glands. Its lateral line is complete throughout the midpoint of the body. The dorsal surface of the head and body may be yellow-brown, olive or grey. Sides are often mottled with vague brown blotches. The underside anterior of pelvic fins may be pale yellow or white. All barbels are dark brown or black, except chin barbels, which may be pale.

During development, eggs are pale in colour and about 3 mm in diameter. Young hatch at 6 mm total length, are yellow in colour initially, but will darken soon after hatching. Once swimming and feeding actively, the young resemble tadpoles with very dark colouration and similar body shape. Growth is rapid, with sexual maturity attained by age 3 (203-330 mm total length) in females. Maximum age in Canada is reported as 6-8 years.”

A. nebulosus was first introduced in the Pacific islands (1877) with the release of 100-200 individuals for angling in Auckland, New Zealand. These introductions resulted in local viable populations by 1885 (Holcík, 1991). Additional introductions for angling and sport occurred in Hawaii in 1893 (Welcomme, 1988).  European introductions occurred concurrently, with individuals from North America introduced to Germany for angling, sport and aquaculture in 1885 (Scott and Crossman, 1973), leading to subsequent intentional and unintentional secondary spread to Poland (1885) (FAO, 1997), the United Kingdom (Bartley, 2006), Hungary (1902) (FAO, 1997), Finland (1922) (FAO, 1997), Belarus (1935) (Reshetnikov et al., 1997) and Bulgaria (1975) (Uzunova and Zlatanova, 2007). Recent introductions (1984) of A. nebulosus from North America to Hubei province and Beijing, China have occurred solely for aquaculture.

Within the United States, A. nebulosus was introduced west of its native Mississippi drainage for angling, sport and aquaculture purposes throughout the 1900s. Introductions within Canada have been concentrated within British Columbia, within the lower Fraser River and lakes within Vancouver Island (Crossman, 1991). The source population, date and reasons for introduction are uncertain for British Columbia populations.

Although A. nebulosus is not considered a quarantined pest, several countries (Switzerland (Wittenberg, 2005); Poland (FAO, 1997); Chile (Welcomme, 1988)) report adverse effects on native fish communities following its establishment. The risk of accidental introduction from aquaculture facilities (see Distribution Table for list of facilities) will be dependant upon the type of facility (e.g., enclosed versus wild pens) and proximity to natural waterbodies. Facilities that are physically connected to natural waterbodies likely pose the greatest risk of introduction. Of these facilities, the effectiveness of physical barriers (e.g., netting, electricity) will determine the likelihood of introduction due to potential escapement.

The risk of introduction of A. nebulosus for angling and sport purposes is dependant upon the goals and regulations of fishery management programs (e.g., the encouragement of non-native fish stocking for sport, or biological control) and the prevalence of unauthorized introductions within areas of potential introduction. Although angling for A. nebulosus within Canada is relatively unpopular (Scott and Crossman, 1973), the initial introduction to Europe and the Pacific islands for angling suggests almost global popularity as a sportfish; therefore, unauthorized introductions for angling purposes may occur.

Natural dispersal of wild populations through drainage networks will likely occur given the species’ moderate swimming ability (Scott and Crossman, 1973). In particular, the ability to travel throughout degraded waterbodies is facilitated by the species’ ability to withstand high water temperatures (up to 37.5°C), industrial pollution, and low oxygen concentrations for prolonged periods (Scott and Crossman, 1973). Over-winter survival is also likely high and may not hinder dispersal success, given the species’ ability to tolerate 0.2 ppm oxygen during winter (Scott and Crossman, 1973).

Natural Dispersal

A. nebulosus is a moderately strong swimmer (Scott and Crossman, 1973) that is capable of surviving degraded, warmwater freshwater environments; therefore, the species will undoubtedly increase its introduced range through natural dispersal within drainage networks. The magnitude of spread will be dependant upon the spatial configuration of potential habitats and their connectivity via drainage networks. Most natural dispersal within the native range of Canada and the USA has occurred at local levels (Fuller et al., 1999).

Vector Transmission

Although A. nebulosus are predated upon by larger fishes, the likelihood of long-distance dispersal resulting from this vector is low. Most piscivorous fishes are unable to utilize A. nebulosus for food due to their sharp, strong dorsal and pectoral spines that may lock into an erect position when predated upon.

Intentional Introduction

Introduction via fisheries (angling/sport purposes) and aquaculture are the dominant long-distance (national; international) vectors. Intentional introductions by fisheries managers may result in long-distance travel events via stocking from source populations. Aquaculture introductions may have similar magnitudes of spread. For example, China (Beijing and Hubei province) stocked A. nebulosus for aquaculture purposes from USA broodstock (Welcomme, 1988). Unauthorized introductions by anglers also has the potential to contribute to local, national or international events but may be constrained by the effectiveness of certain legal restrictions that prohibit import of live organisms across borders.

The brown bullhead (A. nebulosus) is frequently confused with black bullhead Ameiurus melas (southern Canada, USA and parts of Mexico), yellow bullheadAmeiurus natalis (southern Canada, parts of USA), snail bullhead Ameiurus brunneus (southern USA), spotted bullhead Ameiurus serracanthus (southern USA) and flat bullhead Ameiurus platycephalus (southern USA).

A. nebulosus is immediately distinguishable from spotted bullhead by the lack of obvious darkened blotches running the length of the body, whereas a strongly depressed head distinguishes the flat bullhead. Differences between brown, black and yellow bullhead are more conspicuous and concern the morphology of anal rays and pectoral spines. While the yellow bullhead’s anal fin overlaps anterior rays of its caudal fin, both brown bullhead and black bullhead have anal fins that do not reach anterior rays of the caudal fin. Yellow bullhead have relatively smooth pectoral spines; black bullhead possess barbs that point towards head when fins are closed against the body; whereas, the barbs of brown bullhead point towards the caudal fin when closed (Scott and Crossman, 1973; Fuller et al., 1999).

A. nebulosus prefers warmwater habitats within lakes, ponds, rivers and streams. Coincident with its introduction for aquaculture and sport, it may also inhabit artificial channels (e.g., irrigation ditches; canals) that possess suitably warm water for spawning (21.1°C). Aquatic vegetation is usually necessary, as are mud or sand bottoms. The species is benthic and, therefore, persists most frequently in the nearshore area, but has been captured as deep as 12 m in offshore habitats (Scott and Crossman, 1973). The physiological tolerances of the species allow colonization of very warm (up to 37.5°C) waters with low (as little as 0.2 ppm) dissolved oxygen. The species is frequently found within waters containing substantial domestic and industrial pollution, but may bury within mud substrates to avoid continual exposure to pollutants (Scott and Crossman, 1973).

Genetics

Hybridization between brown bullhead and black bullhead may occur where the species frequently co-occur (Scott and Crossman, 1973). No other hybrids are known within North America. A. nebulosus has been sequenced and is described in Arai et al. (2001), Hardman and Page (2003), Hubert et al. (2008) and Sullivan et al. (2006).

Reproductive Biology

The spawning behaviour of A. nebulosus was described in detail by Scott and Crossman (1973): “Spawning occurs in late spring and summer in Canada, when water temperatures reach 21.1°C. Regions lacking a yearly cooling period may have repeated spawning events within a single year. During spawning, one or both sexes clear a shallow nest in mud or sand, usually near aquatic vegetation or other available cover (rocks, stumps). Water depth may be between 15 cm and several meters. Spawning sites are usually contained within protected waters (coves, bays, etc.), with spawning occurring during the day. Male and female circle the nest, caressing with barbels. Following deposition, eggs are pale in colour and coated with mucus, 3 mm in diameter. Between 2000-13,000 eggs may be deposited by mature females. Egg care is provided by one or both parents, during which the eggs are fanned continuously with paired fins to increase oxygen concentrations. In Ontario, Canada, eggs take 6-9 days to hatch at 20.6-23.3°C. Immediately following hatching young are 6 mm in length and pale, but soon darken and swim and feed actively. Of note is the spherical swimming pattern that young will follow, which occurs immediately above one or both of the resting parents, until young are 51 mm total length. Growth is rapid with young reaching 51-122 mm by the end of the first growing season.”

Physiology and Phenology

Physiological specializations allow A. nebulosus to tolerate waters contaminated with organic and inorganic pollutants (Scott and Crossman, 1973). The species may also adapt to extremely low dissolved oxygen concentrations and warm waters (Scott and Crossman, 1973). The specific physical mechanisms of these physiological specializations are unknown. Little documentation exists regarding phenological variation in native and exotic ranges.

By virtue of its strong pectoral and dorsal spines, the adult A. nebulosus is well protected from predation by all but the largest fish predators in their native range in Canada (Scott and Crossman, 1973). Although present in juveniles, the spines are less robust making juveniles more susceptible to predation by fishes with a wider range in size. Within its native range, predators include members of the pike family (Esox spp.) and pike perches (Sander spp.) (Scott and Crossman, 1973). Where introduced on the Gulf Islands of British Columbia in Canada, there are no other piscivorous fishes present and, hence, no native predators to introduced A. nebulosus.

To date, economic impacts resulting from A. nebulosus introductions have not been quantified. In certain cases of wild establishment, A. nebulosus introductions have the potential to hinder local commercial and sport fisheries through competition with target species.

A. nebulosus populations may hinder local native sport fisheries by out-competing target fishes, resulting in reduced angling opportunities and their social impacts. Alternatively, introductions may be encouraged locally if A. nebulosus are favoured for sport. Introduction into previously fishless waters may provide new or valued angling opportunities. Current estimates of social impacts resulting from introductions have not been documented.

A. nebulosus may be detected from shallow waters using conventional fish sampling methods (minnow traps, fyke nets, seines, boat and backpack electrofishers). Once captured, the species may be identified through analysis of meristics and morphometry using the dichotomous key from Scott and Crossman (1973).

Several authors have described A. nebulosus introductions to broad geographical areas (e.g., Welcomme, 1988), but such reports typically lack details of specific biological and economic impacts due to their substantial geographic scope. Therefore, future research should describe the complete biological and economic impacts associated with local A. nebulosus introductions, which would allow for better understanding of the risks associated with intentional or unintentional A. nebulosus introductions. Determining the economic benefits of A. nebulosus aquaculture should also be described, which would allow fishery managers to evaluate its use given the potential for inadvertent biological and economic harm.