Northern wolffish and spotted wolffish recovery strategy: chapter 7

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6. Habitat Identification

Habitat characteristics within much of the area occupied by wolffish are poorly described given the vast area and great depths below the surface that constitutes the range of the species. Knowledge of the habitat associations of wolffish is limited (Simpson and Kulka 2003) and extent of the habitat that is critical for survival remains undefined (Kulka et al. 2004). Known associations are based on the occurrence of wolffish in research trawls and the associated depth and water temperature collected during the survey sets. Seabed classification data (ROXANN) that have been collected since 1992 were used to relate sediment type in the vicinity of survey trawl locations. From these acoustic data, seabed roughness and hardness indices were derived to classify the sediment to categories of mud, sand, sand & shell, shell & pebbles, small rock, hard bottom or undefined (Naidu and Seward 2002, unpubl. data). These studies, reported in Kulka et al. (2004) were undertaken at the center of distribution of the three species on the Grand Banks to Labrador Shelf.

6.1 Habitat Associations

The three wolffish species inhabit a large proportion of Canadian Atlantic waters over a variety of benthic habitats but are at the center of their distribution, reaching highest density and covering the largest area, on the northeast Newfoundland and Labrador Shelf. There they distribute over a wide range of depths, from about 20 m to >1500 m, with A. denticulatus occupying the widest range and A. lupus the narrowest. Temperature is an important feature of wolffish habitat. All three species are associated with a narrow thermal range of above average bottom temperatures (mainly 1.5-5.0°C) and are largely absent where temperatures are <0⁰C. As such, wolffish may be classified as “temperature keepers”; they maintain a similar temperature range by changing their distribution. Given their narrow thermal association, cooling that occurred in the late 1980s-early 1990s may have contributed to the distributional changes observed for wolffish during that period (Kulka et al. 2004).

A common misconception is that the three species of wolffish share a common habitat association residing in rocky crevasses. Kulka et al (2004) indicated that each of the species occupies a somewhat different niche. Key differences between species, described below in more detail, are as follows: A. lupus distributes in more shallow, southern waters more often associated with hard bottoms than the other two species; A. denticulatus spends more time off bottom, and when on bottom at deeper locations are over more diverse bottom types; A. minor inhabits an intermediate niche in terms of depth and temperature.

6.1.1 A. denticulatus

A. denticulatus is a deepwater fish of cold northern seas. At the center of its distribution on the Grand Banks to Labrador Shelf, it is found at depths from 38 to 1504 m (maximum depth surveyed), densest concentrations occurring offshore between 500 and 1000 m (slightly shallower in the warmest months) at temperatures of 2 to 5°C, (Kulka et al. 2004). It inhabits a wide range of bottom types, including mud, sand, pebbles, small rock and hard bottom (Kulka et al. 2004). Unlike other wolffish, A. denticulatus has been found off bottom during both juvenile and adult stages in the northeast Atlantic (Shevelev and Kuzmichev 1990). In the Canadian Atlantic, it spends a significant amount of time in the mid-water as evidenced by a diet comprising a significant proportion of bathy-pelagic (water column below 200 m) and meso-pelagic (upper 200 m) organisms (Roman et al. 2004 and research presently under way). Like other wolffish, it does not form aggregations as dense as some commercial species. Tagging studies by Templeman (1984) suggests that it undertakes only limited migrations. In the northeast Atlantic, the species has been observed defending an area around bait and acoustic tracking showed that the size of that area was quite restricted (Godø et al. 1997). Refer to Table 1 and 2 for further details.

6.1.2 A. minor

The distribution of A. minor is quite similar to that of A. denticulatus except that they seldom inhabit the deepest trenches or as deep along the shelf slope. Observed in waters at depths between 56 and 1046 m, the densest concentrations of A. minor occur between 200 and 750 m and at temperatures of 1.5-5.0°C (Kulka et al. 2004). Similar to A. denticulatus, it inhabits a wide range of bottom types including mud, sand, pebbles, small rock and hard bottom (Kulka et al. 2004). Tagging and other studies indicate that migrations are local and limited (Templeman 1984). A. minor is a benthic feeder consuming a wide variety of echinoderms, crustaceans and molluscs associated with both hard and sandy bottoms. Refer to Table 1 and 2 for further details.

6.1.3 A. lupus

A. lupusis primarily demersal and inhabits shallower depths than the other two species. It is commonly observed near shore out to 918 m (the deepest record of occurrence off the Labrador Shelf) and tolerates temperatures from –1.0°C to 10.0°C. The densest concentrations are found between 150 and 350 m and at temperatures of 1.5°C to 4.0°C. Although this species is found as far north as the Labrador Shelf, highest densities are found on the southern Grand Banks and Scotian Shelf, and is the most common of the three species there and in the Gulf of St. Lawrence. Unlike the other two species, A. lupus is often observed as individuals close to shore by divers. They also form dense concentrations offshore. Movements are limited but seasonal inshore migrations may occur in the spring when mature fish are found in shallow waters at depths of 0 to 15 m. They feed around rocky bottoms on whelks, sea urchins, brittle stars, crabs, scallops and occasionally redfish. Large eggs are laid in clusters on the bottom, often in rocky crevasses, and are guarded by the male. Larvae remain mostly close to the bottom, rarely swimming to the surface and tend to remain close to the site of hatching (COSEWIC unpublished). Refer to Table 1 and 2 for further details.

Table 2. Average bottom depth and temperature by NAFO Division from a) spring and b) fall research surveys at locations where wolffish were captured.

Wolffish were observed to associate with six sediment types (Table 3). The bolded numbers show the largest values for each species for each survey period. In DFO fall research surveys, the average number per trawl was greatest on sand and shell hash sediments for A. denticulatus and A. minor, while the average number per trawl for A. lupus was greatest on rock habitats. All three species were associated with all sediment types except mud, in which only A. minor were captured. During the spring research surveys, the average number per tow for A. lupus was greatest on rock sediments, similar to the fall survey results. However, for both A. denticulatus and A. minor, the greatest number per tow occurred in association with coarse sand sediments. Kulka et al. (2004) also investigated inter-annual differences in the association of wolffish with sediment association.

Table 3. Average catch rates for wolffish from DFO spring and fall research surveys in relation to average bottom type derived from ROXANN data. Highest values are bolded.

These data provide preliminary information on wolffish habitat utilization with respect to three habitat features, temperature, depth and bottom sediment. However, they are insufficient to provide definitive evidence on habitat characteristics that are required to achieve and sustain viable wolffish populations.

6.2 Critical Habitat

Kulka et al. (2004) stated that “direct observations of physical habitat associations for widely distributed, oceanic species, such as wolffish, is problematic. Determining what is critical to survival of a species in an enclosed and directly observable environment such as a marsh or pond is far less complicated than for species that inhabit vast expanses of the unobservable ocean sub-surface.”

Three factors impede the definition of critical habitat in the open ocean in general and for wolffish in particular. First, deficient knowledge of wolffish life history, second, limited information on the influence of multi-scale processes upon wolffish population dynamics, and third the lack of information on acceptable targets for wolffish population abundance and range. Consequently, it is difficult to define critical habitats for wolffish, particularly since each developmental stage may have different requirements that are at present unknown.

Kulka et al. (2004) spatially defined changes in extent of the habitat for the three species on the Grand Banks, northeast Newfoundland and Labrador Shelves by mapping and overlaying the historic and present range of the three species (refer to Figure 5 in that paper). For all three, the area formerly but not presently occupied (potential habitat) occurs primarily on the inner portions of the northeast Newfoundland and Labrador Shelves off the coast of Newfoundland and Labrador. The presently unoccupied area is greatest for A. denticulatus, least for A. lupus.

Given that two of the species, A. lupus and A. minor, are increasing in part of the range and stable elsewhere, and that A. denticulatus is stable over much of its range, it may be that area presently occupied is sufficient to maintain viability. However, reoccupation of all or part of the formerly occupied range would constitute a desirable target and an indicator of recovery, in conjunction with an increase in abundance.

Minimum habitat needed to maintain or increase current population size for the three wolffish species are not known at this time. Initial wolffish habitat data have been collected (Simpson and Kulka 2003, Kulka et al. 2004 - see description of critical habitat above). These data, though inconclusive, give preliminary evidence of wolffish habitat preferences but are insufficient to provide definitive evidence on the habitat characteristics needed to achieve and sustain viable wolffish populations. However, for the reasons stated above, present occupation appears to be sufficient to at least maintain current population size.

6.3 Habitat Trends

Kulka et al (2004) described reductions in distribution of wolffish for the Grand Banks to Labrador Shelf. A. denticulatus occupied 57% of the surveyed area from the Grand Banks to Labrador Shelf, contracting to a low of 19% during 1990-1995, increasing to 23% since 1995. Unoccupied areas or areas previously occupied corresponded to the coldest locations. The area occupied by A. minor also decreased during the period of decline. Area occupied changes were less variable than for A. denticulatus, ranging from 48% to 23%, and currently at approximately 31%. A. lupus underwent the most significant changes in the northern part of the surveyed range, similar to the other two species. Area occupied, previously 55%, declined to a low of 38% and is currently at approximately 56%. The concentration of this species occurring on the shallower part of the Grand Banks underwent relatively little change.

The area occupied index for A. lupus on the Scotian Shelf was lower in the 1990s following a decline in the 1980s, steady at a low level since 2000 (McRuer et al. 2001). In the southern Gulf of St. Lawrence, this index increased during the early 1980s and has remained at slightly higher values since. A. lupus became slightly more concentrated on the Scotian Shelf during the 1980s and has remained so throughout the 1990s. In contrast, in the southern Gulf of St. Lawrence, the distribution increased in the 1980s and has remained marginally more wide-spread throughout the 1990s (McRuer et al. 2001).

Kulka et al. (2004) indicated that temperature is an important feature of wolffish habitat. Changes in temperature have affected distribution, if not abundance. All three species are associated with a narrow thermal range of above average bottom temperatures and absent where temperatures are <0°C. This explains why they do not occur on the northern Grand Bank or the banks northeast of the island of Newfoundland, where sub-zero temperatures occur year round. More specifically in terms of location, the inner portion of the shelf, where all three species underwent their greatest reduction, corresponded to the coldest areas of the range of each of the three species. At their lowest abundance (1990-1995), each of the species was restricted mainly to the warmest locations available along the outer shelf and it was during this period that some of the lowest bottom temperatures were recorded (Colbourne et al. 2004). Kulka et al. (2004) speculated that unfavourable temperatures over a part of the range could have restricted the extent and population size of wolffish.

In terms of sediment type, it is expected that sediments would have remained relatively constant with some alteration brought about by trawling activity over a relatively small part of the area (Kulka and Pitcher 2001).

6.4 Habitat Protection

At the present time, critical wolffish habitat has not been specifically defined and effect of fishing gears on the benthic habitat of wolffish is poorly understood. Thus, no specific protection plans can be put in place for the conservation and protection of wolffish habitat in eastern Canadian waters. Kulka and Pitcher (2001) showed that about 20% of the shelf area of the Grand Banks to Labrador Shelf, mainly on the outer shelf, was trawled annually during the early 1980s. Area trawled dropped to about 5% in the 1990s. It is these areas where alteration of the benthic habitat would have taken place. However, nature and extent of alterations and potential effect on wolffish species is unknown.

7. Ecological Role

Eggs, larvae and juveniles of wolffish are susceptible to predation by a number of species, although for at least one of the species (A. lupus) the eggs are guarded by the adult males until hatching. Adults have fewer predators given their size and substantial teeth. They may also spend a part or their time in rock crevices. The role of each wolffish species as a forage fish is undetermined, though they do appear to be a food source for several species as larvae and young. A. denticulatus in the northeast Atlantic has been observed defending a territory around bait on the bottom from cod and haddock, and acoustic tracking over time showed that the size of that territory was quite restricted (Godø et al. 1997).