Table of Contents
Modeling the transport of lobster (Homarus americanus) larvae and postlarvae in the Gulf of Maine
ABSTRACT: We used a coupled physical-biological model to examine distances between hatching and settlement locations for lobsters in the Gulf of Maine. The physical model is based on a finite element mesh and climatological averages of the seasonally-evolving temperature and density fields. Larval trajectories from coastal and offshore hatching sites (21-224 m deep) were calculated for early, middle and late-season hatching by coupling temperature-dependent development rates and depth (the biological model) to the circulation. Model results showed large spatial differences in larval development times (from 18 to 38 d) and distances transported (19-280 km) for the early hatch. Development time and transport decreased markedly by mid-season at most sites, but strong spatial differences persisted. The eastern Maine coast appears to experience stronger removal and less resupply of larvae than other regions, consistent with observed lower recruitment. Inverse solutions of the model for larvae arriving in mid-coastal Maine indicate that they originate from a broad section of the eastern coast "upstream", with those nearest the shoreline generally traveling the shortest distances. The postlarval stage is neustonic, and a simple inverse model demonstrates that a diurnal coastal sea breeze can contribute substantially to inshore movement during this final planktonic stage. Thus, offshore reproduction may be linked to inshore recruitment.
Fig. 1. The Gulf of Maine and Georges bank study area in the Northwest Atlantic Ocean.
Fig. 2. Transects used for larval releases are numbered 1-10. Each transect has ten release points denoted by asterisks. Shaded regions correspond to depth intervals (m) shown in the scale at right and are the same throughout this paper.
Fig. 3. Locations (open symbols) where sexually mature female lobsters were found by bottom trawl surveys in the Gulf of Maine and Georges Bank. Browns Bank and Nantucket Shoals are not included; see text for details. Depths and shading as in Fig. 2.
Fig. 4. Bimonthly climatological averages of residual current velocities from the circulation model: (a) May-June; (b) July-August; and (c) September-October. Depths and shading as in Fig. 2. Panels d-f show corresponding climatological averages of model surface temperatures (EC, see color bar).
Fig. 5. Composite modeled larval trajectories for (a) early, (b) middle and (c) late hatching periods at each of the ten transects. The scenes are not synoptic: hatching occured later in the north in each scenario and development times depended on temperatures experienced along each trajectory. Larvae were transported until they developed to the end of Stage III, those locations being marked by open circles. Depths and shading as in Fig. 2.
Fig. 6. Average temperature, development time, transit speed, and trajectory length experienced by larvae hatched at the ten transects during three periods in the hatching season. Panels a-c show absolute values for each period according to the model (cf. Fig. 5). Panels d and e show the differences between middle and early-hatching larvae, and late vs. middle hatching larvae, respectively.
Fig. 7. Inverse model results of particle trajectories simulating a fixed larval duration of 28 d and arrival points between Penobscot Bay and Casco Bay, Maine (see Fig. 1). Model "endpoints" are shown by asterisks located along two orthogonal transects from the coast and a connecting transect which follows the 100 m isobath. Predicted trajectories are white; predicted origins (28 d earlier) are shown by open circles.
Fig. 8. Inverse model of neustonic postlarval transport. All model runs begin near the end of postlarval development at a T-shaped arrangement of end-points (asterisks). The top of the "T" is near and approximately parallel to shore along the 20 m isobath, while the stem is orthogonal to the coastline (same as transect 6 in Fig. 2). The predicted beginning of the postlarval stages (i.e., the locations where Stage III larvae molt to postlarvae) are shown by open circles. Panel a shows results with no sea breeze; panel b shows a weak sea breeze with a 10 km offshore extent; panel c is the same as b but with a 20 km offshore extent.
|
Author: Preferred Customer
|