Roughhead grenadier (Macrourus berglax) COSEWIC assessment and status report: chapter 6

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Biology

General

M. berglax is a member of the family Macrouridae (grenadiers or rattails). This family of fishes is one of the most widespread families occurring on the continental slope of the North Atlantic and along the mid-Atlantic ridge. Roughhead grenadiers are slow-growing, late-maturing, and have a long life cycle (Scott & Scott 1988).

Reproduction

Age determination by scales and otoliths proves to be difficult in this species due to the very indistinct annuli but females have been aged up to 25 years (Cohen et al. 1990). This problem is common to many deep-sea fishes and is probably related to their generally low metabolic rates (Smith & Hessler 1974).

Like other members of the Macrouridae, the roughhead grenadier is slow-growing, late-maturing, and has a long life cycle (Scott & Scott 1988). At 7 to 9 years of age females begin to grow faster than males (Murua 2001), and this growth difference increases with age. Murua & Motos (2000) found female roughhead grenadiers in the Northwest Atlantic to mature at 66.7cm (L50), which corresponds to approximately 13-14 years according to their length-age key. Eliassen & Falk-Petersen (1985) found females to mature at 15 years of age in the Northeast Atlantic using direct observations.

Generation time was estimated as female age at maturity plus 1/M, where M is the instantaneous rate of natural mortality. Based on catches in the Spanish 3NO survey, González Costas & Murua (2005) reported values of 0.34 for females and 0.71 for males for Z, the instantaneous rate of total mortality, rather high in light of current knowledge of life history and fishing pressure. These estimates of Z are based on catch curve analysis and will be overestimates if old fish are under-represented in the survey catch due to a tendency to be distributed in waters deeper than those surveyed. Assuming that Z is greater than M due to fishing mortality, a value of 0.2 was assumed for female M, yielding an estimate of 19 yr for generation time.

Savvitimsky & Gorchinsky (2001) found the number of females was close to 50% of the population before 9-10 years of age. At age 12 and older, only females were found. This is a result of females having a longer lifespan than males (Savvatimsky 1994; Murua 2000). The commercial catch at present usually includes ages between 5 and 10, with a peak at 6 years of age (Savvitimsky & Gorchinsky 2001; Junquera et al. 2001).

M. berglaxis a sexually dimorphic species; females grow larger and as discussed above live longer than males. The males also have two large, intrinsic, drumming muscles on the forward part of their swim bladder (Cohen et al. 1973).

M. berglax spawning occurs in winter and early spring and may even extend over an entire year (Eliassen & Falk-Peterson 1985; Scott & Scott 1988; Murua & Motos 2000). One female is estimated to produce 25,000 large eggs, which are laid over a lengthy spawning period and represent a relatively low fecundity for fish of this type (Cohen et al. 1973). Whether the species is itero- or semelparous is unknown. Roughhead grenadier eggs are reported to be pelagic and have a hexagonal pattern membrane (Eliassen & Falk-Peterson 1985). The exact locations of spawning grounds are not certain but they are thought to lie on the southern and southeastern slopes of the Grand Bank (Scott & Scott 1988).

Survival

Zaferman (1992) directly observed grenadiers with a submersible on the North Atlantic Ridge and found their movements directly related to the velocity and orientation of the current. Although the observations were not made on roughhead grenadier, it is generally agreed that they are very slow-moving creatures making them easy potential prey for larger predatory fishes inhabiting the same area. They have been found in the stomach contents of cod and other predatory fish.

Other important sources of mortality are anthropogenic. Roughhead grenadiers are one of two grenadiers to be commercially exploited in the western North Atlantic and they are a significant component of the by-catch in the Greenland halibut fishery (Duran et al. 1997).

Physiology

The roughhead grenadier is generally found in water temperatures between 2.0 and 5.4 ºC but specimens have been retrieved from temperatures slightly below 0 ºC in the Norwegian and Barents seas (Scott & Scott 1988; Cohen et al. 1990; Murua & Motos 2000).

Rattails, like some other deep-sea fish, are known to have specialized swim bladders that function at great depths, and therefore great pressures, in the ocean. The wall of the swim bladder is impermeable to gases. The presence of certain lipids resists the outward diffusion of oxygen (Wittenberg et al. 1980).

Movements/dispersal

Katsarou & Naevdal (2001) found evidence that roughhead grenadier in the North Atlantic do not comprise a single panmictic stock. Instead, there appear to be at least three stock units (West Greenland, East Greenland and Norwegian Sea) each with their own gene pools. An implication of this study is that Canadian populations may be distinct from other populations in the North Atlantic. However, the estimated genetic distances between stock units were low and Katsarou & Naevdal (2001) concluded that the evolutionary significance of these genetic differences is uncertain and possibly low.

Nutrition and interspecific interactions

Roughhead grenadiers are known as non-specialist predators and feed on a wide variety of invertebrates (Cohen et al. 1973; Cohen et al. 1990). The food type consumed by this species is usually directly dependent on the size of the individual fish. Smaller fish feed on small bivalves, starfish, shrimps and polychaetes, which are essentially benthic prey. Larger individuals tend to eat active benthopelagic organisms such as larger bivalves, shrimp, small fish, and squid (Eliassen & Jobling 1985; Scott & Scott 1988). Eliassen & Jobling (1985) found that crustaceans are the most widely consumed prey organisms but fish may make up a considerable portion by weight in the diet of mature females during the summer months in Norwegian waters.

Behaviour/adaptability

The combination of low fecundity, slow growth rates, late maturity, and long population turnover times makes these fish highly vulnerable to population disturbance. Therefore, they are presumed to have low adaptability to sudden change.