Page 10
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-1 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. 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 there by 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".
.