Cusk (Brosme brosme) COSEWIC assessment and status report 2012: chapter 9
Growth and Maturity
Halliday (2006) reported that the only data on Cusk growth in the Northwest Atlantic are those of Oldham (1972), which indicate that age 5 and 11 Cusk are about 45 cm and 70 cmlong respectively. Oldham (1972) calculated that 50% maturity for Cusk in NAFODiv. 4X occurred at age 4.7yr and 43.5 cm for males and at 6.5 yr and 50.7 cm for females. Oldham (1972) also found that males do not grow faster than females although they mature more rapidly. The oldest fish aged was 14.
Maturity at length data collected on the DFO summer and NMFS spring and summer surveys were analyzed using a logistic model to update estimates of the size of maturity. Length and sex were used to explore their relationship with proportion mature. While the DFO data indicated that the interaction between sex and length was significant, the coefficients were similar (*Appendix 1). The NMFS data suggested a stronger sex – length interaction. The DFO and NMFS data indicated that the length at 50% maturity (sexes combined) was about 39 and 42 cm respectively (Figure 14).
Figure 14. Relationship between Cusk proportion mature (sexes combined) and length (cm) based on an analysis of DFO and NMFSspring – summer surveys.
Long description for figure 14
Chart of the relationship between proportion of mature Cusk (sexes combined) and length (centimetres) based on an analysis of summer surveys conducted by Fisheries and Oceans Canada and the U.S. National Marine Fisheries Service. The length at 50 percent maturity is about 39 centimetres for the National Marine Fisheries Service data and 42 centimetres for the Fisheries and Oceans Canada data.
Aging work conducted at the NMFS Northeast Fisheries Science Center (NEFSC) in Woods Hole during 1991 – 95 was made available by O’Brien (2011). Both survey and commercial samples provided 820 otoliths which were used to estimate spring and fall age-length keys. A von Bertalanffy growth model was fit to the data, which provided estimates of L∞, K and T0 of 126.56, 0.110 and 0.673 respectively [1]. This growth curve compares reasonably well with the linear model employed by Oldham (1972) (Figure 15). O’Brien (2011) emphasized that these ages have not been validated and are unpublished and thus these data have not been used further in the analysis of Cusk population dynamics in this assessment. This growth curve and the maturity at length relationship described above suggest that 50% maturity occurs between ages 4 and 5, slightly younger than the estimates of Oldham (1972).
Figure 15. Growth models of Scotian Shelf and Gulf of Maine estimated by Oldham (1972) and O’Brien (2011) respectively.
More recently, radiocarbon bomb dating methods have been used to provide preliminary estimates of the age of Cusk from the Scotian Shelf (Harris and Hanke, 2010). This aging effort has returned older age estimates than the previous Northwest Atlantic studies, including an 82 cm fish aged 39 years. These new data suggest that Cusk may reach maturity by age 10 in contrast to previous estimates of age 4 – 6. These data are currently being further analyzed for publication, which was not available at the time of writing of this report.
In the Northeast Atlantic, Bergstad and Hareide (1996) report lengths at ages 5 and 11 as 35-40 cm and 50-55 cm, respectively. Magnússon et al.. (1997) give an “average growth” curve showing length at age 5 of about 27cm and at age 11 of about 47cm, substantially lower than Bergstad and Hareide (1996) although largely based on their data (Halliday 2006). Cusk in the Northeast Atlantic reach maturity by 40-45 cm when they are 8 – 10 years of age. No significant difference in growth rate has been observed between the sexes (Magnusson et al. 1997). These growth rates are more consistent with the new, preliminary, Northwest Atlantic study than that of Oldham (1972) and O’Brien (2011). Halliday (2006) notes that Cusk are difficult to age (Bergstad et al.. 1998) and it is possible that the apparent faster growth reported in the northwest Atlantic could, at least in part, be due to differences in interpretation of the otolith rings. The northeast Atlantic age readings were based on intercalibrations among expert age readers from several laboratories and are thought to be more reliable (Halliday 2006). In the northeast Atlantic, ages as old as 20 years have been obtained for Cusk of 70-80 cm. Given that Cusk reach lengths greater than 100 cm, longevity could be substantially greater than that. Icelandic bottom trawl surveys catch many Cusk less that 40 cm with modes at 15 cm that correspond to age 2 fish, and 7 – 8 cm that correspond to age one (Bergstad et al.. 1998).
Based upon the examination of life history data of over 1200 fish species in FishBase, Froese and Binohlan (2000) provide a relationship between L∞ and the maximum observed age, LMAX (equation 1), which for Cusk is 115 cm. This provides an estimate of 111.4 cm for L∞.
Ln(L∞) = 0.044 + 0.9841 * Ln(LMAX) (1)
A rearrangement of the von Bertalanffy growth equation (equation 2), along with estimates of L∞, LM (length of maturity of 42 cm) and TM (age of maturity of either 5 or 10 years) and assuming T0 is about 0.7 (based on NMFS growth model and age data), estimates of K, the Brody growth coefficient, were either 0.11 (assuming TM = 5) or 0.05 (assuming TM = 10).
K = -(Ln(1 – Lm / L∞)) / (TM – T0) (2)
Given the uncertainty in age estimates, further research is required to elucidate the growth dynamics of Cusk.
Reproduction
Oldham (1972) reported a range in fecundity of 100,000 eggs in a 56 cm fish to 3,927,000 eggs in a 90 cm Fish.
Observations of the occurrence of ripe and spawning maturity stages in commercially landed fish during 1964 indicated that most Cusk spawning took place in the last half of June but that the spawning season lasted from May to August (Oldham 1972). Catches of Cusk eggs in Scotian Shelf ichthyoplankton Program (SSIP) surveys during the late 1970s – early 1980s confirm Oldham’s results, with eggs abundant in plankton net hauls in June and July with smaller quantities taken in May and August and occasional catches as late as September (Figure 16). Also, port samplers examining catches from the western Scotian Shelf and Gulf of Maine have observed Cusk in spawning condition as early as March (Harris and Hanke 2010). Similar ichthyoplankton surveys conducted by the USA in the Gulf of Maine – Georges Bank area at about the same time (Berrien and Sibunka 1999) found eggs from March to November but most occurred in May-June, slightly earlier than in Canadian waters. Cusk eggs were observed largely over the western Scotian Shelf and Gulf of Maine in areas where adults were caught in the summer bottom trawl survey.
Figure 16. Distribution of Cusk eggs on the Scotian Shelf based upon SSIP sampling during 1978 – 82 (from Harris et al.. 2002); scale in eggs /m3 with + designating no. eggs caught.
Long description for figure 16
Six map panels (one for each month from April to September) showing the distribution of Cusk eggs on the Scotian Shelf based upon SSIP (Scotian Shelf Ichthyoplankton Program) sampling during 1978 to 1982. Eggs are abundant in plankton net hauls in June and July with smaller quantities taken in May and August and occasional catches as late as September.
Cusk were caught in the vicinity of Emerald Bank in the bottom trawl surveys during the same period that the SSIP surveys were conducted (Figure 6). Very few Cusk have been caught in the bottom trawl surveys in more recent years. This suggests a spawning component of the population may have been lost.
Ichthyoplankton surveys by the USA confirm that eggs occur throughout Gulf of Maine waters, i.e. in 5Y as well as 4X, and on the slopes of Georges Bank, including its southern edge (Berrien and Sibunka 1999). These data also show an apparently disjunct spawning area at 36-38°N, with eggs occurring at this southern location persistently over several months. In addition, USSR ichthyoplankton surveys on Flemish Cap in 1978-83 caught Cusk eggs in late April and in May (and in one case in March) and one larva (Serebryakov et al. 1987). Thus, egg distributions within the main area of adult distribution are widespread, consistent with Oldham’s conclusion that there are not clearly defined, discrete spawning grounds. Nonetheless, maps of egg distributions suggest that the northeastern Gulf of Maine is an area of particularly high egg abundance. It appears that there are also discrete spawning populations on Flemish Cap and in the Mid-Atlantic Bight off Virginia (Halliday 2006).
Eggs, which are about 1.1-1.5 mm in diameter, are pelagic. Larvae hatch at about 4 mm and remain in the upper part of the water column until about 50 – 60 mm, at which time they become benthic (Collette and Klein-MacPhee 2002). Larvae have been captured from Emerald Bank south to Long Island in DFO and USA ichthyoplankton surveys. Larvae are distributed widely over banks (Colton and St. Onge 1974, Halliday 2006) but in the same general area as the eggs, and have been observed in association with the jellyfish Cyanea (Colton and Temple 1961).
The duration of the early life history pelagic phase is not reported but likely depends on water temperature and is presumably comparable to other Gadids – about 1 – 4 months (COSEWIC 2003). The location of benthic nursery grounds is not known. Few Cusk under 30 cm have been caught in Canadian summer RV surveys (Halliday 2006) or in longline surveys.
Demersal juveniles and adults remain strongly associated with the substratum and do not swim up into the water column (Bigelow and Schroeder 1953, Collette and Klein-MacPhee 2002). As adults, Cuskare described as slow-moving, sedentary and solitary, and do not form large aggregations or schools (Svetevidov 1948, Wheeler 1969, Cohen et al.. 1990).
Predator – Prey Associations, Condition and Natural Mortality
Halliday (2006) reports that Spiny Dogfish (Squalus acanthias) is the most frequently recorded predator of Cusk. Predation by Winter Skate (Leucoraja ocellata), Atlantic Cod, White Hake (Urophycis tenuis), Atlantic Monkfish (Lophius americanus), Cusk-eel (Lepophidium cervinum), Sea Raven (Hemitripterus americanus), Summer (Paralichthys dentatus)and Windowpane Flounder (Scophthalmus aquosus) has been observed in US waters (Collette and Klein-MacPhee 2002). There are occasional records of predation by Cod and Atlantic Halibut on the Scotian Shelf (Harris and Hanke 2010, Harris et al.. 2002). Cusk are a very small part of the diet of Grey Seals (Halichoerus grypus) (Bowen et al.. 1993, Bowen 2011) although there is little information available on the diet of Grey Seals in NAFODiv. 4X, where Cusk primarily reside. So this could reflect a geographic sampling problem (Bowen 2011) and Grey Seals could be a predator in this area.
Northwest Atlantic observations of Cusk diet are limited due to the propensity of their stomachs to evert when brought to the surface (Scott and Scott 1988, Bergstad 1991). The available data indicate that food consists primarily of Crustacea, particularly crabs, shrimps and euphausiids (krill), fish (species not recorded) and echinoderms (brittlestars) (Langton and Bowman 1980, Bowman et al.. 2000, Harris and Hanke 2010, Harris et al.2002). Off Norway, diet is similar, consisting of crustaceans (shrimp and Norway Lobster), small fish (Norway Pout (Trisopterus esmarkii), Atlantic Halibut and redfish (Sebastes sp.)) and polychaete worms (Magnússon et al.1997).
Fish condition can give some indication as to the feeding success or otherwise of Cusk. However, data on weight at length of individual fish were collected irregularly on DFO summer bottom trawl surveys with no collection during 1986-98 and very small numbers of observations recently. The annual weight of a 65 cm Cusk is shown in Figure 17. During the late 1970s and 1980s the weights were generally above 2.8 kg. In more recent years the weights have been between 2.6-2.8 kg. It is difficult to state with confidence whether this is an important trend in fish condition.
Figure 17. Change in weight (kg) of 65 cm Cusk during 1970 – 2010.
Long description for figure 17
Plot showing the weight of 65-centimetre Cusk during 1970 to 2010. In the late 1970s and 1980s the weights were generally above 2.8 kilograms. In more recent years the weights have been between 2.6 and 2.8 kilograms.
There is no information on the natural mortality (M) rate of Cusk. It is, however, likely a relatively long-lived species (see above) and thus natural mortality could be expected to be similar to other large Gadiformes such as Cod and Haddock, i.e.in the vicinity of 20% per year under normal conditions. Pauly (1980) provides a relationship (equation 4) between natural mortality (M), K, the Brody growth coefficient and mean annual temperature experienced by a fish (T).
Ln(M) = -0.0152 – 0.279 Ln L∞ + 0.6543 Ln K + 0.4634 Ln T (4)
Mean average bottom temperature during 1970 – 2010 in NAFODiv. 4X, based upon the DFO summer survey is 7.2 °C. While this is during the summer, the annual average is not expected to be much lower, in the order of 6 °C. This provides an estimate of M of 0.14 or 0.09 depending upon whether K is 0.11 (age 5 maturity) or 0.05 (age 10 maturity). This compares with estimates based on the formula (M = 1.5 x K) of Jensen (1996) of 0.17 and 0.08 for age 5 and age 10 maturation respectively. It is important to note that adult Cod and Haddock on the eastern and western Scotian Shelf have experienced relatively high rates of natural mortality (in the order of 0.5 – 0.8) in the late 1980s and through the 1990s (Worcester et al. 2009). The extent to which the stocks of these and other species such as Cusk that occur in the western Scotian Shelf/Gulf of Maine area experienced a coincident elevation in natural mortality rates is not known.
Generation Time
The current aging data for Northwest Atlantic Cusk suggest that the age of 50% maturity is about 5. If natural mortality is about 0.14, generation time is TM+1/M = 12.1 years. If on the other hand, the age of 50% maturity is about 10 and M about 0.09, generation time is 21.1 years. The shorter generation time of 12.1 years will be used in this assessment because the new aging data have not been corroborated by further analyses.
Cusk appear to be a relatively slow moving, sessile species that does not undergo extensive local movements, seasonal, or spawning migrations. In the Northwest Atlantic, spawning occurs over banks similar to other Gadid species, with likely retention of passively drifting eggs and larvae by gyres with subsequent localized settling and active movement to preferred rocky habitat areas. Fragmentation between isolated populations within Canada does not appear to be an issue given the highly localized distribution of the species in the Gulf of Maine area.
There has been limited consideration of the role of Cusk in aquatic food webs off the east coast of Canada. Ecopath modelling (Bundy 2004, 2005) of the Scotian Shelf considered Cusk along with other demersal piscivores such as White Hake and Sea Raven. This modelling indicated that juveniles and adults have trophic levels of 3.95 and 4.2 – 4.4 respectively. No special feeding relationships were noted. As noted above, Atlantic Cod and Atlantic Halibut are likely predators.
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