Prepared by: EFH Technical Team
(Excerpted from the 1999 Habitat Annual Review Report) Sections:
Age-1 and Older Juvenile Habitat and Movements
Age-1 juveniles are found during day and night in shallow inshore waters, including locations with moderate to high wave exposure (Keats 1990). Older juveniles are generally distributed farther away from shore than 0-group and 1-group cod and at depths >25 m. Age-1 associate to a greater degree with rocky substrate and fleshy macroalgae or bottom dominated by sea urchins and coralline algae (Keats et al. 1987; Keats 1990; Gotceitas et al. 1997). The association with a macroalgal canopy seems to be more one of refuge from predators than feeding purposes (Keats et al. 1987; Gotceitas et al. 1995; Gotceitas et al. 1997). They congregate in small groups near boulders and in large crevices. In Newfoundland bays, age-1 cod have been collected within a slightly narrower temperature range, 1-160C, than demersal 0-group fish (-1.7-i70C) (Methven and Bajdik 1994).
At dusk during sunmier and autumn seasons, age-i and older juveniles move shoreward into warmer water feeding areas where the young-of-the-year cod are concentrated. The attracting stimulus appears to be the periodic influxes of early settled cod (Keats 1990; Clark and Green 1990; Methven and Bajkik 1994). Age-1 cod have usually been found feeding until dawn primarily on mysids and ganimarid ampbipods; however, when they become about three times larger than settled age-0 juveniles, they begin cannibalizing the demersal 0-group cod (Grant and Brown i998a). By late fall, the earliest age-0 settlers may be large enough to begin intracohort cannibalism on the late settlers, as has been noted in waters of Iceland (Bogstad et al. 1994). When abundance of older juveniles is high, mortality may increase on young-of-the-year because of competition and predation from conspecifics (Grant and Brown 1998a).
Age-1 cod have also been observed feeding on plankton after moving inshore in spring (Keats Ct al. 1987) as well as resting near bottom in shallow water at night (Keats and Steele 1992). In the latter situation, age-i were not feeding and analysis of stomach contents indicated daytime foraging on planktonic crustaceans leading the authors to speculate that post-transitional feeding on benthic invertebrates might be patchy in space and time. Where, when, and to some extent what yearlings eat is likely related to trade-offs between predation risk and food availability.
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Juvenile cod may utilize the intertidal zone for feeding purposes although there is no mention of this in recent studies. Earlier, an underwater television camera mounted on a herding fence recorded 423 "young" Atlantic cod (no size given), and Atlantic tomcod (Microgadus tomcod), which were sometimes indistinguishable from cod, as well as six, 3 0-40 cm (age-3 to 4) cod moving up and down a beach, either with or against tidal current, during daytime between June and October in Passamaquoddy Bay, New Brunswick (Tyler 1971). Of eight fish species observed undertaking these movements, the cod! tomcod combination ranked third, behind only winter flounder (Pseudopleuronectes amercianus) and Atlantic herring (Clupea harengus) in their use of the intertidal zone.
Diel Dfferences in Abundance
Keats (1990) found one- and two-year-olds 16 times more abundant at night than during the day while making SCUBA transects at a depth of 5-9 m MLW. Methven and Bajdik (1994) were able to seine age-i cod throughout the year but only at night in a cove of Trinity Bay, Newfoundland, whereas age-i were caught both day and night by Grant and Brown (1998a) in a different cove of the same embayment. An explanation for the difference in catch of yearling cod between the two studies may be related to sampling techniques. The first study employed a 9 m seine pulled from a maximum depth of 1.2 m (no bridle). The second utilized a 30 m seine deployed by small boat 50 m from shore and pulled by towropes thereby encircling age-i cod inhabiting a slightly greater depth range.
Researchers studying young cod recognized that gear avoidance occurred during daylight, but avoidance was secondary to diel activity in explaining abundance differences between day and night catches for both age-0 and the older juveniles (Methven and Bajdik 1994; Gibson et al. 1996; Methven and Schneider 1998; Grant and Brown 1998b). Abundance of age-i peaked in the shore zone from August-November and again in April-June period, but was much reduced in winter (temperature <00C) indicating withdrawal to deeper habitat. The offshore movement by young cod was also reported in Passamaquoddy Bay, Bay of Fundy (MacDonald et al. 1984).
Juvenile Winter Habitat and Activity
Juveniles inhabit progressively deeper water and associate with coarser substratum as they grow and mature, especially in winter (Keats et al. 1987). Age-1-4 cod were observed at 18 to 1 50dm from submersible vehicles during April (-1 0C at 25-7 Sm), Placenta Bay, Newfoundland (Gregory and Anderson 1997; Gregory et al. 1997). They found that 80% of two-to four-yearolds were associated with rock, boulders, and high bathymetric relief (cliffs) and often maintained fidelity to such features including crevices in rocks. They exhibited significant increases in swimming speed with increasing distance from structure. Yearling cod showed no such connection, 59% of those observed were primarily over gravel and low relief with the fish appearing to rely on cryptic patterns to remain undetected. Macroalgae was neither avoided or preferred by either group. Age-1 and ages 2-4 co-occurred laterally and vertically throughout the study area most abundantly at depths of 60-120 m. Juveniles did not appear to undertake a diet movement shoalward during the winter! early spring season. However, onshore movements may be initiated during March and April after ice break-up and coincident with nearshore water temperature of~2-30C. The same temperature prompts offshore movements in late autumn (Metbven and Bajkik 1994).
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Sonic tagged age-3 cod (28-33 cm) rested almost exclusively in rocky areas at night during winter (Clark and Green 1990). Between June and September, however, individuals were active nocturnally and wide ranging (>3 km]day), moving daily between deep (30 m) cold water, where they were inactive in rocky areas, to shallow (<15 m) sandy substratum where they were active at night in relatively small feeding areas (~540-2,580 in2). When the water column became isothermal in September, age-3 cod remained in the shallow water during dayligtit leading researchers to speculate that the switch from nocturnal to diurnal feeding might be an antipredator strategy, i.e., to avoid being cannibalized at night when adult cod are seasonally active in relatively shallow water. Other common predators of juvenile cod off Newfoundland are pollock (Pollachius virens) and shortfinned squid (Jllex illecebrosus).
Spatial Depth Gradient of Juveniles
For three years following stock collapse, Methven and Schneider (1998) undertook extensive sampling of the Newfoundland coastal zone to a depth of 55 m and by a variety of gears. Finding consistent spatial and diel changes in catch across gears, they interpreted results as characteristic of cod distribution. Catch rate of age-0 cod was inversely related to depth each year, highest at night, and higher at 4-7 m, the center of 0-group distribution during autumn. There was a sharp decrease in catch rate at 20 m (Schneider et al. 1997). Demersal age-0 cod were found almost exclusively alongshore within the northeastern coastal bays of Newfoundland; yearlings extended further offshore and older juveniles were widely distributed on the continental shelf confirming an ontogenetic pattern of movement to deeper water with increasing size. Age-dependent distribution was also obvious from trawl station catches on survey transects extending from the coast to hundreds of kilometers offshore (Dailey and Anderson 1997). When the stock was more robust, demersal age-0 cod were distributed more widely onto the shelf.
The only coastal region of eastern Canada where the seasonal pattern of distribution for young cod appears to be different is the coastal portion of southern Gulf of St. Lawrence where water temperatures might be too warm during summer months (Hanson 1996). Fine scale distribution studies with trawls found that cod did not occupy water 2-12 m deep along shores of Pnnce Edward Island during summer. They were mostly absent from shallow waters (<20 m deep) in the Miramichi estuary and the contiguous Shediac Valley coastal shelf during any time of yeat. Yearlings and 2-year-olds, but not age-0 cod, were almost exclusively found in 15-35 m depths of the Gulf from June to early October before joining older age-groups in an extensive migration to deep (>100 m) offshore water for winter.
The spatial depth gradient of juvenile cod from all other areas of eastern Canada seems consistent with published information from the Northeast Atlantic. The depth of highest age-0 cod abundance using a beam trawl off the British Isles was 6 m (Riley and Parnell 1984). Greatest density of age-i cod sampled with gill nets off Greenland was <20 m (Hansen and Lehmann 1986; Hovgard and Nygaard 1990). Acoustic surveys off the Norwegian coast showed most juveniles at depths <35 m and highest densities of demersal 0-group cod very close to rocky shores where the research vessel could not survey (Olsen and Soldal 1989).
Density-dependent Habitat Use and Mortality
Contraction or expansion of geographic range with decreasing or increasing population size has
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been observed in a number of cod stocks including the Labrador-East Newfoundland complex and southern Gulf of St. Lawrence stock. In the latter region, the area occupied by age groups 3-8+ cod increased as abundance increased (Swain and Wade 1993). In comparison to the older cod, age-3 were more spatially restricted at low population size, their range expanded more slowly as abundance increased, and changes in relative density among parts of the Gulf were smaller between years of low- and high-abundance. Younger juveniles were thought to experience less severe competitive pressures for food or wider variation in habitat quality than the older age-groups.
A behavioral theory applied to explain the pattern of geographic distribution is density-dependent habitat use. This hypothesis was applied to young cod in coastal habitats (Olsen and Soldal 1989) where catches of post-settlement juveniles showed a high degree of small-scale spatial consistency regardless of cohort size. In years of high year-class abundance, density increases to an upper limit in the most suitable habitat and as the fitness of individuals occupying the prime sites declines due to intraspecific competition, diffusion to and use of suboptimal habitat expands.
Accordingly, at low population size, individuals occupy habitat with high basic foraging and protective suitability.
The theory was tested for the Labrador-East Newfoundland stock complex for which contraction has been confirmed for adult cod at low stock size (Taggart et al. 1994; Atkinson et al. 1997). Catches of age-groups 0-2 were analyzed from 1959-64 and 1992-94 at a series of fixed sampling sites extending over 1,500 miles of Newfoundland coastline (Schneider et al. 1997). In years of low cohort size, contraction did not occur in coastal habitats, i.e., density of juvenile cod was independent of area within the occupied <20 m depth range. They noted that sampling sites with high densities in some years had low densities in years of high abundance, an observation inconsistent with spillover theory in good years.
In support of density-dependent theory, high post-settlement densities of age-0 cod were found in eelgrass beds of Trinity Bay, Newfoundland, during 1994 and 1995, years of good and bad year-classes, respectively; however, a significant increase in abundance in less suitable no-eelgrass habitat was noted in 1994 when settlement strength was high (Grant and Brown 1998a). The high 1994 densities in less-utilized no-eelgrass habitat during a year of high abundance would be consistent with the hypothesis of density-dependent habitat use or selection. The researchers acknowledged that their observations were on a small temporal and spatial scale. Re-analysis of the fixed sampling site juvenile catch data from Newfoundland showed a stronger recruitment signal from a small number of sites visited frequently than the entire set of sites (Lngs et al. 1997). The 1994 year class was ranked significantly stronger than the three previous year-classes following stock collapse in a broad-scale study (Anderson and Dalley 1997). On the other hand, there was no evidence of fewer settled 0-group juveniles anywhere along the coast in 1995 relative to the 1992-94 year-classes (Smedbol et al. 1998).
For a number of cod stocks, variability in year class strength is usually determined in the larval
stage and attenuated by density-dependent juvenile mortality (Myers and Cadigan 1993a).
Biological processes that may result in density-dependent mortality would include:
(1) competition for food with mortality resulting from increased predation or starvation;
(2) intercohort cannibalism; (3) predators switching to abundant year-classes; and (4) a
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circumscribed area of prime juvenile habitat with those settling surviving while others do not, resulting in a upper limit to the number of survivors regardless of egg/larval production. This mechanism could involve food limitation and/or increased predation risk outside a prime nursery area. It presumes mechanisms maintaining a relatively constant density such as territorial behavior or some other form of density-dependent habitat utilization.
Notwithstanding the study by Schneider et al. (1997), many of the research results discovered and re-confirmed by scientists undertaking the studies summarized herein, describe or infer habitat mediated density-dependent mortality rates. These mechanisms systematically affect cod survival rates from the post-settlement pelagic stage well into the demersal juvenile stage. Annual variation in survival rates on these life stages may be more important in affecting year class size than survival in pre-settlement stages (Sissenwine 1984). This suggests that the nearshore bottom habitat may become a potential bottleneck to year-class size particularly in areas where the availability of the most suitable habitat might be low.
Summary of Research
In shallow (< 5 m) coastal areas of eastern Canada, pelagic juvenile cod settle onto various subtidal habitats in several periodic pulses beginning in May. Space use is highly localized and primarily focused on the need to acquire food and avoid predators. Relative to fulfilling both needs, activity periods, substrate choices, and interactions with members of same species and others are critical. Diurnal feeding in intercohort schools aids location of patchily distributed plankton and provides protection against predators. Site fidelity and nightly concealment in all habitats, except sand, minimizes interactions with cannibalistic age-i cod that move shoalward at dusk to feed. The spatial pattern of age-0 cod distribution is altered by post-settlement mortality such that abundance among bottom habitats matches substratum complexity: cobble/gravel _rock reef> eelgrass _ macroalgae> sand. Of bottom habitats studied, eelgrass confers a significant advantage in growth to age-0 cod. Significantly reduced predation risk also occurs if eelgrass stems are above a threshold density and/or they are associated with cobble bottom. Eelgrass meadows are highly utilized as nursery habitat both spatially and temporally through at least mid-summer. The transition to a demersal existence occurs at a length of 6-10 cm and is marked by a switch to benthic prey foraged at dawn and dusk.
The distribution of age-0 cod in autumn is centered at depths of 4-7 m MLW with a sharp drop off at 20 m. In late autumn! early winter, age-0 lose site fidelity and disperse to deeper water where they congregate primarily over gravel and low relief cover.
Older juveniles inhabit progressively deeper water and associate with coarser, hard-bottom features as they grow. Seasonal inshore movements are usually associated with nocturnal feeding. the availability of better information about the actual distribution and spatial extent of seagrasses and hard bottom habitats.
Inshore Gulf of Maine Activities
This proposed HAPC would be located entirely within the waters of the states of Maine, New Hampshire and Massachusetts. The only jurisdiction the Council has over activities occurring in this proposed HAPC is for the fishing activities of federal permit holders who also fish in near-shore state waters. Most activities that would occur within the proposed HAPC fall under the management jurisdiction of state agencies.
There are a range of alternatives the Council could consider to minimize the potential adverse impacts of fishing gear and practices within the proposed HAPC. One option would be to impose no additional restrictions on the fishing activities of federal permit holders within this area. At the other end of the range of options, the Council could consider restricting all fishing activity by federal permit holders within this proposed HAPC. The Council could also consider restricting only such fishing activities of federal permit holders that employ bottom-tending mobile fishing gear.
To have a meaningful effect on the activities that may adversely impact the proposed HAPC, it may be necessary for state fishery agencies to restrict the use of certain fishing gears and practices. The Council can make specific recommendations to state fishery agencies and encourage them to protect EFH and HAPCs. Many seemingly benign activities -- ranging from hand raking of bay scallops, which are almost always found in eelgrass beds, to subtidal aquaculture operations -- may require more scrutiny than they have been given to date and the Council could recommend that state fishery agencies consider these activities in light of the importance of this habitat.
Although depths <9 m are rarely fished with large bottom-tending mobile fishing gear, small boat commercial fishermen use dredges to fish for sea scallops and sea urchins. Such gear might be more properly restricted to waters deeper than 9 meters. When nearshore fisheries commenced for these species, they were reported to be initially undertaken by SCUBA divers but now dredging is the most popular method. Hand gathering may be a more appropnate method for harvesting relatively sessile resources in sensitive shallow habitats. The Council could recommend that state fishery agencies consider options to close this shallow coastal zone to some or all bottom-tending mobile fishing gear.
Other traditional fisheries undertaken close to the littoral zone, such as dragging for blue mussels (Mytilus edulus), raking Irish moss (Chondrus crispus), or hand digging quahogs (Mercenaricz mercenaria) may not be a problem based on the substratum occupied. It is possible that these types of activities may be benign to critical habitat for age-O cod.
The Council could consider recommending that state fishery agencies review the potential impacts of these types of activties and, if necessary, consider options to minimize their impacts on the proposal HAPC. Even if no specific measures arc proposed, the HAPC designation would create a higher level of review for non-fishing related activities during the NMFS EFH consultation process.
Age-1 cod, while co-existing in all but the shallowest depths with young-of-the-year, are many times more abundant in the shore zone at night than during the day apparently attracted thereby the presence of periodic influxes of post-larval pelagic juvenile cod. Competitive advantage accrues to the largest and earliest settling juveniles especially those finding coarse substratum with vegetative cover. Those less favored must disperse from feeding patches more often thereby accepting a lower rate of food intake in order to avoid detection and capture. As Tupper and Boutilier (1995b) hypothesized: "one habitat might supply the population with a greater number of smaller recruits, each with a somewhat lesser chance of survival, while another habitat supplies fewer, larger recruits, each with a relatively high chance of survival".
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The trade-offs between habitat use and frequency of feeding in the face of predation risk are processes consistent with density-dependent habitat use and mortality. Although empirical evidence of density-dependent usage off Newfoundland is contradictory, stock size/recruitment may not yet be large enough following the northern cod stock collapse to induce significant density-dependent effects on a large spatial scale. Nevertheless, behavioral research details ways age-O juveniles respond to spatial heterogeneity, the consequences for fitness through utilization of resources, and the intraspecific competitive effects which emphasizes the importance of habitat availability and quality in determining recruitment success.
* End of Part 1
To Part 2: Gulf of Maine Coastal Marine Environment and Juvenile Cod Distribution