Lewis S. Incze

Pre- and post-settlement processes in recruitment of the American lobster

Abstract In censuses conducted between 1989 and 1993 the cause of consistent local differences in benthic recruitment of the American lobster ( Homarus americanus Milne-Edwards) to coastal sites in Maine was unclear. Field experiments were conducted to assess the role of pre- and post-settlement processes in causing high and low extremes in recruitment on opposite sides of an outer coastal island in our study area. The west side of the island has some of the highest population densities measured in New England. Our results indicate that postlarval supply determines these differences in recruitment between the two sites. Standardized replicate cobble plots deployed on each side of the island ruled out habitat differences as an explanation for these differences, because they exhibited the same east-west difference in recruitment as the natural habitat. We also ruled out differing rates of post-settlement loss because we recovered previously marked and released settlers in equal numbers in similar plots from both sites. The distribution of neustonic postlarvae and hydrographic evidence indicate that wind-driven surface transport produces an asymmetric postlarval supply to the two sides of the island during the settlement season. Differences in the degree of asymmetry from year to year correspond to differences in the magnitude of eastward transport. We also conducted experiments at the site receiving high recruitment to assess whether new recruits or older year classes were near the saturation of cobble habitat for these animals. The combination of saturation seeding trials, using hatchery-reared lobsters, and weekly counts of natural recruits and immigrants suggests that lobsters may become increasingly subject to crowding as they grow. Movements away from the initial settlement site, probably in part caused by crowding, tend to smooth the inequality in population density which is set initially by postlarval supply.

Mesozooplankton of Shelikof Strait, Alaska : abundance and community composition

Zooplankton was sampled in Shelikof Strait and some of the surrounding shelf waters from March to October 1985, and then in Shelikof Strait during spring 1986–1989, using 150 and 333 μm mesh nets. The median integrated abundance of copepods in spring was 1–2 orders of magnitude greater in the Strait (maximum depth >250 m) than over shallower, adjacent regions of continental shelf. Zooplankton fauna was a mixture of oceanic and continental shelf taxa, and among the copepods was strongly influenced by abundance of two deep-water species,Neocalanus plumchrus andMetridia pacifica Seasonal development of the copepod community in Shelikof Strait followed similar patterns over the 5-year period. Biomass of copepods showed some large interannual differences related mostly to abundance of the oceanic taxa. Large interannual differences were found for some of the other zooplanktonic taxa as well, but there did not appear to be any correspondence between the patterns shown by these groups and those shown by the copepods. Although euphausiids were not quantitatively sampled, the taxonomic composition and relative abundance and seasonality of species within this group are reported.

Interactions of a mesoscale patch of larval fish (Theragra chalcogramma) with the Alaska Coastal Current

Abstract Walleye pollock, Theragra chalcogramma, form dense aggregations during a brief spawning period from late March to mid-April in Shelikof Strait, Alaska. Spawning produces a large (order 20 × 50 km or more) “patch” of eggs at depth (generally > 150 m), and hatching larvae often produce a “patch” in the upper 50 m. Patches can be defined as coherent features using graded concentration isopleths, and the mean concentration within patches has been observed to be as much as 68 times (for eggs) and nearly 6 times (for larvae) the background concentration. Larval patches drift southwestward and have been identified for about 30 days after hatching in some years. Data are presented for spawner biomass and for early life stages, as available, for 1981, 1983 and 1985. When comparing 1985 with 1981 (2 years with the best coverage), spawner biomass and mean egg concentration within the patch declined concordantly. Larval concentrations about 10 days after hatching differed widely, however: concentrations in early May 1985 were more than an order of magnitude lower than expected. Unlike either 1981 or 1983, no larval patch could be identified in late May 1985; this appears to be attributable to changes detected earlier in the month. The “apparent” mortality rate for a 10-day period after hatching in 1985 was about 0.50 d−1 greater than in 1981. Larval feeding conditions can be excluded as a likely cause of this interannual difference, but predation and advection cannot be. Our findings emphasize the short time period over which significant changes can occur, as well as additional sampling which must be done in future studies. We show that part of the emerging 1985 larval year class could have been removed by cross-channel disturbances in the flow field through Shelikof Strait.

Distributions of copepod nauplii and turbulence on the southern flank of Georges Bank: implications for feeding by larval cod (Gadus morhua)

Abstract The vertical distributions of copecod nauplii and water properties were sampled at well-mixed and stratified sites on Georges Bank using a pumping system, CTD and in vivo fluorometer during a four day period in late May 1992. At each stratified station at least one sample was taken within the thermocline and the fluorescence maximum, which usually co-occurred. Well-mixed sites had low average concentrations of nauplii, ca 41 −1 , and showed little variation of abundance with depth. Stratified sites had from 4 to 16 times the integrated (0–50 m) abundance of nauplii compared to well-mixed sites and showed strong vertical patterns of distribution. Maximum concentrations of nauplii, up to 1601 −1 , were associated with the thermocline at 7 of the 9 stratified stations. At the two remaining stratified sites the naupliar maximum was in the upper mixed layer, sampled at 5 m depth. The encounter rate between early feeding cod ( Gadus morhua ) larvae and their naupliar prey was calculated with and without turbulence. Turbulence was estimated from two sources: wind stress in the upper layer (calculated from wind observations during our cruise) and tidal shear in the lower layer (estimated initially from a tidal mixing equation). We ultimately replaced the lower layer estimates with turbulence data from a series of measurements made in 1995. The latter are more robust and had the advantage of providing dissipation rates for the pycnocline as well as the lower layer. Theory predicts an increase in encounters between a predator and its prey with the addition of turbulence parameters into standard models of encounter. We combined turbulence profiles with the vertical distribution of nauplii to examine the potential contribution of turbulent kinetic energy to predator-prey encounter rates at various depths in stratified and mixed water columns. Our calculations suggest the following increases due to turbulence at stratified sites on Georges Bank during our cruise: from 34 to 219% in the upper mixed layer, depending on wind speed and depth; approximately 8% in the pycnocline; and approximately 110% below the pycnocline. Mixed sites experience increases of at least 110% (tide only), but greater increases (118–192% in this study) occur when the wind blows because of the combined (spatially overlapped) effects of wind and tidal mixing at these sites. The absolute values for encounter rates and their modification by turbulence are sensitive to a number of assumptions in the models. We used a series of stated assumptions to generate estimates that range from 0.6 to 26.5 prey h −1 , depending on geographical location, physical forcing and depth.