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Auke Bay Laboratory (ABL)

  photo of a giant grenadier
Figure 1.  A giant grenadier caught during the 2004 AFSC sablefish longline survey.

Groundfish Assessment Program

Grenadiers in Alaska

Grenadiers (family Macrouridae) are deep-sea fishes related to hakes and cods that occur worldwide in all oceans. Also known as rattails, they are especially abundant in waters of the continental slope, but some species are found at abyssal depths. At least seven species of grenadier are known to occur in Alaskan waters, but only three are commonly found at depths shallow enough to be encountered in commercial fishing operations or in fishery surveys: giant grenadier (Albatrossia pectoralis) (Fig. 1), Pacific grenadier (Coryphaenoides acrolepis), and popeye grenadier (Coryphaenoides cinereus). Of these, giant grenadier has the shallowest depth distribution and the largest apparent biomass, and hence is by far the most frequent grenadier caught in Alaska.

For management purposes, grenadiers in Alaska are presently considered unspecified species in both the Gulf of Alaska and the Bering Sea/Aleutian Islands Fishery Management Plans. This means that for these fish, there are no limitations on catch or retention, no reporting requirements, no official tracking of catch by management, and that no stock assessments have been done. However, because of concern that grenadiers may in the future be moved to a specified status and require a formal stock assessment, and also because of the importance of these fish in the continental slope ecosystem, work began in 2004 to assemble data on grenadiers that would be necessary in future stock assessments. The work consisted of analysis of existing survey and fishery information on grenadiers and initial field observations and sample collections concerning fecundity and maturity of giant grenadier.

Information from AFSC trawl and longline surveys shows that giant grenadiers comprise virtually all the grenadiers found in waters less than 800 m depth; Pacific and popeye grenadier only become common at depths greater than 800 m. Furthermore, the surveys indicate that giant grenadier are the most abundant fish on the continental slope at depths of 400-1,000 m in all surveyed areas of Alaska except the eastern Gulf of Alaska. As such, they have a significant role in the slope ecosystem and are important predators in this habitat. Surveys also indicate that male and female giant grenadier have different depth distributions. Nearly all the fish at depths of less than 800 m are females, whereas males are found in deeper waters.

Although there has been little or no directed fishing for grenadiers in Alaska, substantial numbers are taken as bycatch and discarded in the sablefish and Greenland turbot longline fisheries. None of the discarded grenadiers survive, as the pressure difference experienced by the fish when they are brought to the surface from deep water invariably causes death. In the past, fisheries observers have not identified grenadiers to species. However, as the longline fisheries operate mostly in depths less than 800 m, it can be surmised that giant grenadier comprise nearly all of the grenadier taken. Based on observer data, estimated annual catches of giant grenadier in Alaska may have ranged between 13,000 and 21,000 metric tons (t) in the years 1997-2001. If true, these estimates indicate that more giant grenadier were caught and discarded in Alaska than the amount of sablefish that was taken during these years.

The large biomass of giant grenadier in Alaska may be able to support this level of catch, but the reported longevity and slow growth of this species makes it susceptible to overfishing. Furthermore, a high proportion of the catch is likely female because mostly female giant grenadier live at the depths where the commercial fishery operates. Disproportionate removal of females by the fishery could put stocks of giant grenadier at greater risk. One possible mitigating factor that may protect giant grenadier from overfishing is that a substantial portion of its population may inhabit depths greater than 1,000 m, where they are presently safe from any fishing pressure. These deep waters could act as a de facto reserve to replenish giant grenadier removed by the fishery in shallower water. Further analyses of fishery and survey data for giant grenadier are needed, as well as additional biological studies, to better determine the population dynamics of this species.

To obtain more biological information on this species, female giant grenadier were sampled for spawning condition, sexual maturity, and fecundity during one leg of the 2004 AFSC sablefish longline survey in the Gulf of Alaska. Visual examination of the ovaries suggested that the overall population in this region may have a very protracted spawning period and that spawning is probably not a synchronous event. Individual fish were in various stages of ovarian development; some were ripe and spawning, whereas others were in very early stages. Determination of size at 50% sexual maturity was unsuccessful, as very few fish were found to be immature. To obtain the complete range of fish sizes needed for a size of maturity analysis, sampling with an alternative gear such as trawls will likely be necessary. All ovaries were preserved, and those in an advanced developmental stage will be used to determine estimates of fecundity.

By David Clausen


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