Chen et al,: Population structure of Homarus americanus in the Gulf of Maine 



249 



has expanded to nearshore waters (from 4.8 km to 32.2 

 km from shore). There is also a deepwater fishery for 

 lobster that occurs farther from shore. 



An optimal and effective fisheries management plan 

 requires a high-quality stock assessment, which, in 

 turn, is dependent on data collected from fisheries- 

 dependent and fisheries-independent survey programs. 

 One of the most important pieces of information used 

 in a lobster stock assessment is the abundance index 

 which is derived from the fisheries-independent bottom 

 trawl surveys conducted by NMFS. Because trawling 

 is difficult in coastal waters, few sampling stations in 

 the NMFS survey are located inshore, specifically along 

 coastal Maine from which the majority of the lobster 

 landings are landed. Previous studies have also indi- 

 cated that inshore habitats are critical to the lobster 

 fishery. Thus, it is critical to develop an inshore survey 

 program that can cover the waters off coastal Maine. 

 The data collected from such a program can overcome 

 the problem of a lack of inshore coverage in the NMFS 

 survey. An inshore survey and the NMFS survey can 

 complement each other for a more complete abundance 

 index of the lobster stock in the Gulf of Maine. 



In order to identify the importance of the DMR in- 

 shore survey, we compared the differences between the 

 NMFS survey and DMR inshore survey in 1) the size 

 composition of survey catches and their temporal trends; 

 2) temporal trends in abundance indices; and 3) average 

 size and weight of lobsters. From our comparative study, 

 we determined whether the lobster stock in the Gulf of 

 Maine has a size-dependent inshore-offshore distribution 

 and whether it is essential to include both sampling pro- 

 grams in a stock assessment to adequately describe the 

 population dynamics of lobsters in the Gulf of Maine. 



Methods and materials 



Like the NMFS surveys, the inshore trawl survey is 

 conducted during the spring and fall of each year. It has 

 a stratified random design modeled after the NMFS and 

 Massachusetts Department of Marine Fisheries (MADMF) 

 surveys. The design has four depth strata (9-37 m, 37-64 

 m, 64-100 m, >100 m [its outer boundary roughly delin- 

 eated by the 12-mile or 22-km limit]) and five regions 

 based on oceanographic, geologic, and biological features. 

 The fourth stratum was added in the spring of 2003. It 

 expands the coverage area to equal the area in federal 

 waters covered by the Atlantic States Marine Fisheries 

 Commission (ASMFC) and allows some overlaps between 

 annual inshore trawl survey area and the NMFS survey 

 area. It also slightly reduces the sampling pressure in 

 the shallower strata, which has been of concern to fixed- 

 gear fishermen in the past. To randomize the survey area 

 (-13,720 km-), each depth stratum is divided into 1-nmi 

 (i.e., 1.852 km) sampling grids. A target of 100 stations 

 is selected for sampl-ng in each survey, resulting in a 

 sampling density of about 1 station /137 km-. This density 

 compares to the sampling density of the NMFS survey (1 

 station/892 km-) and Massachusetts' survey (1 station/ 



65 km'-). The number of stations per stratum is allocated 

 in proportion to the area of each stratum. 'When a station 

 is encountered that cannot be towed, an alternate tow is 

 selected nearby over similar depth. 



Trawl design considerations for the survey included 

 effectiveness of the gear for sampling the complex bot- 

 tom in the Gulf of Maine and approximate comparabil- 

 ity with previous and ongoing surveys, such as that 

 performed by the NMFS. Net tapers are cut to permit 

 the shape of the net to be of maximum height, while 

 allowing the net to remain tight on the bottom. The net 

 is shackled from the footrope to the frame by using two 

 0.95-cm shackles on a banded wire that runs parallel 

 with the footrope. Heavy rubber wing bobbins retard 

 bottom wing lift. The top leg is constructed of 5.1-cm 

 mesh overall and has a 1.3-cm mesh liner in the codend. 

 Doors are 7.5 Bison doors. Attached to the 21.3-m-long, 

 1.59-wide footrope is a roller frame. The 3.05-m-wide 

 bosom section is made up of 20.3-cm rubber disks on 

 15.2-cm centers, and there are eight evenly spaced 

 toggles. The spacing is maintained by small 10.2-cm 

 cookies strung between the disks. Chain sweeps were 

 not used. The headrope is 17.4 meter in length. 



Environmental data, including temperature and salin- 

 ity profiles, wind, sea state, and weather, were collected 

 at each station. A standard trawl tow, 20 minutes in 

 duration, was made at each station. Shorter tow times 

 were accepted under certain circumstances, such as the 

 presence of fixed gears or untrawlable bottom in the 

 survey pathway. Tow speed was maintained at 2.1 to 2.3 

 knots (i.e., 3.9 km/h to 4.3 km/h) and tow direction was 

 oriented toward the tidal current whenever possible. All 

 sampling was conducted during the day. After each tow, 

 the net was brought aboard and emptied onto a sort- 

 ing table. All individuals were identified and sorted by 

 species. All lobsters were immediately separated and 

 processed while the rest of the catch was sorted. Total 

 weights (by sex), carapace length (mm), shell condition, 

 presence and stage of eggs, V-notch condition, and inju- 

 ries to the lobsters caused by the trawl were recorded. 

 All lobsters were measured and the data were recorded 

 in electronic format for analysis and made available on 

 compact disk (CD). The data were geo-referenced and 

 incorporated into geographic information systems (GISj 

 for analysis. 



A no. 36 Yankee bottom trawl has generally been 

 used in NMFS bottom trawl surveys. The trawl net 

 is towed at approximately 3.5 knots (i.e., 6.5 km/h) 

 for 30 minutes at each station. The survey is based 

 on a stratified random design. Strata are defined ac- 

 cording to water depth, latitude, and historical fishing 

 patterns. Within each stratum, stations are assigned 

 randomly; the number of stations allotted to a stratum 

 is in proportion to its area (approximately one station 

 per 892 km'^). Specifications for NEFSC standard no. 36 

 Yankee bottom trawl are defined by the NMFS.' 



^ NMFS (National Marine Fisheries Service). Website: http:// 

 www.nefsc.noaa. go v/femad/ecosurvey/mainpage. [Accessed 

 August 2004.] 



