Volunteer-based monitoring of juvenile American ...

Mar. Freshwater Res., 2001, 52, 1103–12

Volunteer-based monitoring of juvenile American lobster, Homarus americanus Sara L. Ellis and Diane F. Cowan The Lobster Conservancy, PO Box 235, Friendship, Maine 04547, USA. Email: [email protected], [email protected]

Abstract. The primary objective of the Juvenile Lobster Monitoring Program is to develop a time series of abundance and distribution of juvenile American lobsters. Between 1997 and 2000, trained volunteers quadrat sampled 1-m2 quadrats along fixed 20-m transects monthly from May through October at 24 intertidal sites in the Gulf of Maine. Lobster carapace length ranged from 4 to 82 mm (34.6 ± 0.26 mm, n = 1874). Monthly densities of early benthic phase (≤40 mm CL, EBP) lobsters ranged from 0 to 1.50 lobsters/m2 (0.31 ± 0.010 lobsters/m2, n = 1417). Peak EBP densities coincided with peak substrate temperatures recorded in situ at low tide. Interregionally, EBP density was highest in Massachusetts and lowest in Penobscot Bay, corresponding with relative regional substrate temperatures. Mean lobster carapace length was greater, and EBP density lower, in eelgrass than in rocky habitat. In a 1999 bay-wide survey of Penobscot Bay, Maine, lobsters were detected in outer, but not inner, regions. A strong correlation between abundances at intertidal and subtidal sites (r = 0.86, P < 0.001, n = 17) indicates similar patterns of abundance in the two zones. A volunteer work-force allows cost-effective long-term research on juvenile lobsters over a wide geographical area.

Extra keywords: abundance, distribution, ecology, long-term monitoring

Introduction The American lobster, Homarus americanus, is the single most important species to the fisheries of New England (NEFSC 1996a). Effective management of this resource requires an understanding of processes that affect abundance of all life stages and the relationships between these life stages. Consensus is growing that recruitment to the fishery is likely to be influenced by the abundance of new lobsters entering the population each year (Wahle and Incze 1997; Steneck and Wilson 2001 this issue), yet the long-term quantitative measurements of juvenile or adult abundance that could be used to test this hypothesis are generally lacking. Although the National Marine Fisheries Service has conducted bottom trawl surveys since 1963, because of gear selectivity these surveys measure abundance of adult lobsters but not of juveniles (Burns et al. 1983; NEFSC 1996b). The American lobster ranges from the Strait of Belle Isle, Newfoundland, to Cape Hatteras, North Carolina, and is most abundant in relatively shallow coastal zones (ASMFC 2000). It spends its first six to eight weeks of life in three pelagic larval stages, followed by a postlarval stage that settles out of the water column, making the transition from pelagic to benthic life. Recent years have seen a surge of © CSIRO 2001

effort to study recently settled and early juvenile lobsters in the wild (e.g., Hudon 1987; Able et al. 1988; Heck et al. 1989; Wahle and Steneck 1991; Cowan 1999). Various methods, including SCUBA-based visual surveys and suction sampling, have been used to document abundance and distribution of earliest juvenile stages in shallow subtidal areas at depths between 5 and 10 m below mean low water (reviewed by Lawton and Lavalli 1995). Early stages of juvenile lobsters can also be found in shallower water, at the subtidal–intertidal interface (Herrick 1895; MacKay 1926; Templeman and Tibbo 1945; Krouse and Nutting 1990; Cowan 1999), the most landward margin of lobster distribution. Cowan (1999) was the first to quantify the abundance of juvenile lobsters in the lower intertidal zone. This work led to the design of a low-cost, long-term sampling program to study the settlement, abundance, distribution, growth, and movement of newly settled and early juvenile lobsters (Cowan 1999; Solow et al. 2000; Cowan et al. 2001 this issue). One of the main questions that the resultant Juvenile Lobster Monitoring Program (JLMP) addresses is whether future lobster landings within the Gulf of Maine can be predicted from trends in annual abundance of juveniles. Determining patterns of juvenile lobster abundance within the Gulf of Maine requires long-term sampling over 10.1071/MF01194 1323-1650/01/081103

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a wide geographical range. SCUBA-based subtidal sampling methods are labour and resource intensive, making it difficult to sample many sites in a given year (Steneck and Wilson 2001 this issue). The relative ease of sampling in the intertidal zone makes it possible to sample frequently throughout the year (Cowan 1999; Cowan et al. 2001 this issue), and the simplicity of the JLMP methodology is amenable to widespread use by trained volunteers. Volunteer networks frequently provide valuable information to the science and management of environmental resources (e.g., Audubon bird counts, Monarch butterfly migration studies, and Chesapeake Bay Water Quality Network). Well-known too are groups like ‘Riverkeepers’ and ‘Baywatchers’ that have grown in number within the United States. The rise of ‘citizen science’—the organized and informed participation by nonscientists in habitat- and species-saving activities — represents a form of data collection that has been shown to play a critical role in the gathering of long-term data on wildlife and habitats (Youth 2000). Volunteers have been participating in the JLMP since 1995 under the supervision of the authors (DFC, SLE). The work-force provided by citizen volunteers allows costeffective coverage of a wide geographical area, which because of logistical and financial constraints, could not easily be covered by conventional methods. The data collected by volunteers in the JLMP are forming the basis of a time series of abundance and distribution of juvenile lobsters around the Gulf of Maine, which will be used to estimate and detect changes in the abundance of juvenile lobsters both temporally and spatially. These data will ultimately be used to test the hypothesis that trends in juvenile abundance can be used to predict recruitment to the fishery. In the present paper, we present the methodology used to conduct volunteer-based research on juvenile American lobsters. To establish the value of the method, we present results of data collected between 1997 and 2000. We report on size distribution and abundance of juvenile lobsters, both temporally and spatially, and test for correlations between lobster abundance and substrate temperature. We compare lobster abundance among sites and regions, and in inner and outer areas of Penobscot Bay, a large embayment in Maine. We look for interannual trends in abundance over a fouryear period in Casco Bay, Maine. Because the intertidal zone represents the landward margin of lobster abundance, we test for similarity between patterns of abundance in intertidal and subtidal zones. Methods Study sites Between 1997 and 2000, volunteers were trained to quadrat sample for juvenile lobsters along fixed transects at 24 intertidal sites. Sampling

Sara L. Ellis and Diane F. Cowan

Fig. 1. Location of 24 intertidal lobster nursery sites in the Gulf of Maine monitored 1997–2000 to track the abundance and distribution of recently settled and juvenile lobsters, Homarus americanus. sites were distributed along the Gulf of Maine coastline to cover a wide geographical range (Fig. 1). Sites were categorized as belonging to five regions from north-east to south-west: Penobscot Bay (PBME), Casco Bay (CBME), southern Maine (SME), New Hampshire (NH), and Massachusetts (MA) (Fig. 1). Four original survey sites were established in 1997, and eight additional sites were added in 1998, three in 1999, and nine in 2000 (Table 1). Choice of study sites was determined by three main factors: geographical location, habitat availability, and the presence of juvenile lobsters. Sampling sites were concentrated in shallow coves protected from the open ocean, with rocks large enough to provide shelter for lobsters but small enough to be overturned by hand. On Vinalhaven Island in PBME, volunteers sampled in both rocky habitat and eel-grass beds, allowing a comparison of lobster sizes and abundances in differing habitats. Data collected in eel-grass habitat were excluded from regional, interregional, and overall analyses, because the size distribution and abundance of lobsters in eel-grass were significantly different from those in nearby rocky habitat (see Results). Data collection Volunteers for the JLMP were recruited through a series of regional presentations by the authors (DFC, SLE). Before taking on responsibility for a monitoring site, each volunteer was given hands-on training in the field and provided with a training handbook as a reference. The handbook included facts on lobster biology, instructions and illustrations of monitoring techniques, illustrations of relevant marine organisms, and sample data sheets. Handbooks were printed on ‘Rite in the Rain’ waterproof paper (J. L. Darling Corp.) and held in sturdy three-ring binders for use in the field. All volunteers in Maine and New Hampshire were listed on special research permits issued by the Maine Department of Marine Resources and New Hampshire Department of Fish and Game, which allow individuals to handle sublegally sized lobsters. (The state of Massachusetts did not require a special research permit.) Volunteers sampled monthly at specific sites during spring low tides, from May through October. This sampling season was chosen with the goal of sampling during months of highest lobster abundance, as judged from observed seasonality (Cowan 1999; Solow et al. 2000;

Overall

Massachusetts

New Hampshire

Southern Maine

Casco Bay, Maine

Penobscot Bay, Maine

Region

Isle au Haut Vinalhaven-rocky Matinicus South Thomaston Port Clyde Allen Island Cundys Harbor Yarmouth Island Gun Point Little Harbor Mackerel Cove Jaquish Island Potts Pt. Basin Pt. Chebeague Broad Cove Zeb Cove Wells York New Castle Odiorne Point Lanesville Manomet Marblehead

Site

Longitude 68°38.9′ 68°49.8′ 68°53.8′ 69°07.2′ 69°14.7′ 69°18.6′ 69°53.5′ 69°55.5′ 69°56.7′ 69°59.9′ 70°00.1′ 70°00.1′ 70°01.1′ 70°02.6′ 70°07.5′ 70°12.1′ 70°13.0′ 70°34.0′ 70°39.0′ 70°42.9′ 70°42.9′ 70°29.8′ 70°32.0′ 70°50.6′

Latitude 44°03.2′ 44°02.3′ 43°51.1′ 44°01.3′ 43°55.9′ 43°52.6′ 43°47.7′ 43°47.0′ 43°46.0′ 43°43.4′ 43°43.3′ 43°43.0′ 43°44.5′ 43°44.4′ 43°42.8′ 43°34.2′ 43°35.5′ 43°16.3′ 43°08.9′ 43°03.5′ 43°02.4′ 42°40.4′ 40°53.5′ 42°30.6′ 1997–2000

1999, 2000 1998, 1999, 2000 1999, 2000 1999, 2000 1998, 1999, 2000 1998, 1999, 2000 1997, 1998, 1999, 2000 1998 1998, 1999, 2000 1997, 1998, 1999, 2000 1997, 1998, 1999, 2000 1997, 1998, 1999, 2000 1998, 1999, 2000 1998 2000 2000 2000 2000 2000 2000 1998, 1999, 2000 2000 2000 2000

Years sampled

1934

74 68 5 46 132 221 135 28 122 181 58 202 60 24 5 71 33 30 13 65 113 90 2 156 4490

220 113 153 272 300 230 379 31 280 405 322 381 220 76 68 100 64 24 54 86 394 133 76 109

Lobsters Quadrats (n) (n)

4

6 22 36 9 8 7 5 19 6 4 8 16 9 26 34 27 17 29 26 11 20 10 30 11

Min. CL (mm)

82

61 70 50 82 56 70 76 60 69 55 53 76 70 53 53 71 56 80 60 60 65 75 61 80

Max. CL (mm)

34.6

29.9 43.0 43.6 46.0 34.6 39.5 29.2 34.3 34.9 26.0 27.4 34.8 30.0 34.0 42.6 38.3 38.5 44.6 37.7 35.7 34.6 35.8 45.5 35.1

Mean CL (mm)

0.26

1.44 1.33 2.56 1.84 0.67 0.67 0.99 1.91 0.93 0.76 1.34 0.64 1.67 1.38 3.79 1.02 1.87 2.65 2.94 1.33 0.71 1.19 15.50 0.85

SE CL (mm)

1874

73 62 5 46 128 219 134 27 120 178 56 184 53 23 5 71 33 30 13 59 109 89 2 155

CL (n)

Table 1. Summary of sites and lobster characteristics Summary of size and abundance of juvenile lobsters, Homarus americanus, at 24 intertidal monitoring sites in the Gulf of Maine, 1997–2000. Regions are arranged from north to south, and sites within regions are arranged from east to west. CL = carapace length

Volunteer-based monitoring of juvenile American lobster 1105

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Table 2. Site summary for intertidal/subtidal comparison Locations of corresponding intertidal and subtidal lobster-monitoring sites in Penobscot Bay, Maine, used to test for correlation of EBP lobster abundance (Fig. 9) Intertidal sites (this study)

Latitude

Longitude

Subtidal sites (Steneck and Wilson 2001)

Latitude

Longitude

Isle au Haut: Moore’s Harbor Deer Isle: Fifield Pt. Castine Vinalhaven: Lane’s (rocky) Matinicus: Condon Cove Islesboro E: Loranus Cove Islesboro W: Grindle Pt. North Haven: Tartank Cove Searsport: Moose Pt. Rockport: Glen Cove South Thomaston Port Clyde Allen Island

44°03.2′ 44°09.6′ 44°23.8′ 44°02.3′ 43°51.1′ 44°15.1′ 44°17.0′ 44°08.2′ 44°25.9′ 44°08.4′ 44°01.3′ 43°55.9′ 43°52.6′

68°38.9′ 68°42.0′ 68°48.8′ 68°49.8′ 68°53.8′ 68°54.8′ 68°56.4′ 68°55.3′ 68°56.6′ 69°04.9′ 69°07.2′ 69°14.7′ 69°18.6′

Isle au Haut: Moore’s Harbor Deer Isle: Stinson Cape Rosier Vinalhaven E. Matinicus: West Point Isleboro E: Hewes Pt. Isleboro W: Gooseberry Pt. North Haven: Stand-in Pt. Searsport Rockport: Glen Cove Tommy Island Metinic Allen Island #2

43°52.2′ 44°10.1′ 44°19.5′ 44°03.7′ 43°52.0′ 44°18.3′ 44°17.6′ 44°06.3′ 44°26.3′ 44°08.0′ 44°01.1′ 43°52.6′ 43°51.7′

68°54.2′ 68°42.9′ 68°49.6′ 68°46.6′ 68°54.2′ 68°53.0′ 68°56.2′ 68°57.0′ 68°51.6′ 69°04.3′ 69°06.5′ 69°07.9′ 69°19.5′

Cowan et al. 2001 this issue). Sampling occurred during 2-h periods centred around low tides predicted to be more than 0.3 m below mean low water. The protocol was designed for teams of two; one individual sampled and the other recorded data. Environmental data including temperature, salinity, and weather conditions were recorded at the beginning and end of each monitoring session. Air, substrate, and water temperatures were measured in degrees Celsius with a digital Checktemp Pocket Thermometer (Hanna Instruments), and surface salinity was measured in parts per thousand with a temperaturecompensated hand-held salinity refractometer (SPER Scientific). Volunteers sampled 1-m 2 quadrats along 20-m fixed transects along the water’s edge (Cowan 1999). Transects were chosen to include rock cover in all quadrats; in cases where a 20-m transect would have included quadrats without rock cover, the transect was split into two 10-m transects. Transects were fixed through use of natural landmarks at one or both ends of the transect or by triangulation with onshore landmarks. A qualitative description of each quadrat included an estimate of the percentage rock cover and substrate type to the nearest 25% and a record of the presence of marine organisms and macroalgae. Movable rocks in quadrats were overturned one at a time, and presence of organisms beneath the rocks was noted. When lobsters were detected, the following data were recorded: carapace length (CL, from the rear of the eye socket to the posterior margin of the carapace), total length (from the tip of the rostrum to the tip of the telson), handedness (right or left crusher), sex (for lobsters measuring >15 mm CL), condition of appendages ( e . g ., missing, regenerating, damaged), shell condition (hard, brittle, or soft), rock dimensions (length by width by height), and depth of water under the rock. Carapace length was measured to the nearest millimetre with sliding stainless-steel Apprentice Pocket Calipers (General Tools Manufacturing), and total length with calipers or a ruler. Sliding stainless-steel pocket calipers were chosen over vernier calipers for their simplicity of use (e.g., unambiguous metric markings), which minimized the possibility of measurement error. Each lobster was measured in situ and immediately returned to its shelter. Data collection was standardized by use of two types of data sheets: general data sheets for daily and summary environmental information and quadrat data sheets for recording substrate descriptions, lobster characteristics, and presence of other marine organisms. Data sheets were designed with simplicity in mind. For example, in instances where choices occur, such as ‘left’ or ‘right’ crusher claw or ‘male’ or ‘female’, choices were provided on the data sheets.

Data analysis After sampling was completed, all data sheets were sent to and archived at The Lobster Conservancy. Data were entered into an Excel-based electronic database. As a data-quality control, data on lobster size were used only if the ratio of total length to CL was approximately 3:1, a ratio based on the expected morphometric relationship. Direct detection of settlement is difficult because postlarvae generally undergo ecdysis within hours or days of settling. Further, because young-of-the-year lobsters represent only a single year class, their abundance is often low and undetectable in regions of moderate to low lobster settlement (Steneck and Wilson 2001 this issue). Because recent research has shown a strong positive relationship between the abundance of recently settled lobsters and early benthic phase (EBP) lobsters (Steneck and Wilson 2001 this issue), we used relative abundance of EBP lobsters as an index of settlement. Early benthic phase lobsters were defined as measuring ≤40 mm CL (Wahle and Steneck 1991). Abundance was calculated as number of EBP lobsters per square metre sampled and was calculated on several different scales. At the smallest scale, i.e., by site, monthly abundance was calculated as the mean and standard error of the number of EBP lobsters found per square metre. For broader-scale comparisons of abundance, such as by region or by month, EBP densities per square metre were pooled by category for calculation of mean and standard error. In a time-series analysis within Casco Bay, data from the four sites established in 1997 were pooled for calculation of mean monthly density within each sampling season. For an overall comparison of lobster abundance with monthly substrate temperature, mean substrate temperatures from all sites and months were pooled. Within the Penobscot Bay region, we conducted two regional analyses. On the basis of earlier studies of juvenile lobster abundance in large estuarine embayments (Wahle 1993; Cowan 1999), we expected to find lower juvenile abundance in inner than in outer regions of Penobscot Bay. Our null hypothesis was that sites in inner and outer portions of the bay would not differ in juvenile abundance. For a test of this hypothesis, 10 additional intertidal sites were monitored in inner regions of Penobscot Bay throughout the 1999 sampling season. In the second regional analysis in Penobscot Bay, we tested for a correlation between EBP lobster abundance at 13 corresponding intertidal and subtidal sites, 1998–2000 (Table 2). The goal of this

Volunteer-based monitoring of juvenile American lobster

analysis was to determine whether patterns of abundance in the intertidal zone were representative of lobster abundance in the subtidal zone. Subtidal data were provided by Steneck and Wilson (2001 this issue), who sampled for lobsters using SCUBA-operated venturi suction devices, vacuuming 0.25-m2 quadrats to a substrate depth of about 20 cm (sampling protocols of Wahle and Steneck 1991 and Wahle and Incze 1997; Steneck and Wilson 2001 this issue). Although intertidal data were collected from May through October, subtidal data were collected only in July and August, so comparisons were based on July and August intertidal surveys by volunteers. Ten sites were represented in 1 year, three sites in 2 years, and one site in 3 years. All data were analysed with SPSS (Version 8.0). Means are presented ± one standard error (SE).

Results Between 1997 and 2000, quadrat sampling for lobsters was conducted by trained volunteers at 24 intertidal sites along the coast of the Gulf of Maine (Fig. 1). A total of 1934 lobsters were detected in 4490 square metres (Table 1). Of these, 1874 were measured for CL (Table 1).

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14, P = 0.03). Because of the difference in size distribution and abundance in eel-grass, data from the eel-grass site were excluded from all other analyses of CL and lobster abundance. Carapace length of lobsters ranged from 4 to 82 mm (mean 39.4 mm; Table 1; Fig. 2). All lobsters were below minimum legal size (83 mm CL). By site, mean CL ranged from 26.0 to 54.8 mm (Table 1). By region, mean carapace length ranged from 32.1 ± 0.33 mm CL (n = 884) in Casco Bay to 42.0 ± 1.46 mm CL (n = 43) in southern Maine (Fig. 3a). Regions differed significantly in size-frequency distribution of lobsters (Kruskal-Wallis test, P < 0.001). Casco Bay ranked lowest, suggesting that more small lobsters were detected in this

Size distribution and habitat use Mean dimensions of rocks that sheltered lobsters were 38 × 28 × 15 cm (n = 1586). In 92.9% of cases, lobsters were found solitarily under their rock shelters. Two lobsters were found together in 6.2% of cases, 3–6 were found together in the remaining 0.9%. In a side-by-side comparison of rocky intertidal habitat with eel-grass intertidal habitat on Vinalhaven Island, lobsters were larger in eel-grass (54.4 ± 2.06 mm CL, n = 30) than in rocky habitat (43.0 ± 1.33 mm CL, n = 62) (t = 4.8, df = 90, P < 0.001). Monthly abundance of EBP lobsters was significantly lower in eel-grass (0.06 ± 0.028 per m2) than in rocky habitat (0.17 ± 0.042 per m2) (t = –2.2, df = 400 Early Benthic Phase ( < 40 mm CL)

mean = 34.6 ± 0.26

n = 1,874

Frequency

300

200

100

0 4

8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84

Carapace Length (mm)

Fig. 2. Size-frequency distribution of lobsters (Homarus americanus) at 24 intertidal study sites in the Gulf of Maine, 1997–2000. Lobster sizes are grouped in 4-mm increments. The size range of early benthic phase lobsters is indicated by a dotted line.

Fig. 3. Size distribution of lobsters (Homarus americanus) at 24 intertidal sites in the Gulf of Maine, 1997–2000 (n = 1874) (a) by region and (b) by month (regional abbreviations: PBME = Penobscot Bay, Maine; CBME = Casco Bay, ME; SME = southern Maine; NH = New Hampshire; MA = Massachusetts; see Fig. 1). In these box plots the 25th, 50th, and 75th percentiles are shown by lines at the bottom, middle, and top of each box, respectively. The largest and smallest values that are not outliers are shown as thin horizontal lines; open circles show outliers and asterisks show extreme values.

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region than elsewhere (Fig. 3a). Regions and months differed significantly in mean carapace length (2-way ANOVA, region F(4) = 24.2, P < 0.001; month F(5) = 6.3, P < 0.001; interaction F(17) = 2.7, P < 0.001; Figs 3a and 3b). Post-hoc tests indicated that mean lobster size was significantly lower in Casco Bay than in all other regions (least-significant-difference (LSD) test, P < 0.001; Figs 3a and 3b), and mean lobster size across all regions was significantly lower in August and September than in July (LSD test, P < 0.001; Fig. 3b). Patterns of abundance and distribution

0.50

18

0.45

16

0.40

14

0.35 12 0.30 10 0.25

Substrate T ( C)

Mean EBP density (lobster per m 2 )

To look for seasonal patterns, we pooled data on EBP abundance and substrate temperature by month for all years. Overall, density of EBP lobsters showed a strong seasonal pattern that tracked substrate temperature (Fig. 4). Mean substrate temperature generally increased from May to August, then decreased, whereas mean EBP abundance generally increased from May to July, then decreased (Fig. 4). Within years, monthly densities of EBP lobsters at monitoring sites ranged from 0 to 1.50 lobsters/m2 (mean 0.31 ± 0.010 lobsters/m2; n = 1417). Monthly EBP densities at monitoring sites were weakly but significantly correlated with local substrate temperatures (r = 0.32, P < 0.001, n = 213). To determine whether any sites were nursery ‘hotspots’, we determined the highest monthly EBP densities for each site in any year (Fig. 5). The two highest monthly EBP densities were recorded at Little Harbor, CBME, and Marblehead, MA (1.50 and 1.48 EBP lobsters/m 2 , respectively). These abundances were 50% higher than those at the next four closest-ranked sites (Jaquish Island

and Gun Point in CBME, and Port Clyde and Allen Island in PBME, all with highest monthly EBP density = 1.00 lobsters/m2). The top six ranked lobster nursery sites were located in all regions except New Hampshire and southern Maine. We further examined patterns of abundance between regions by comparing EBP densities in 2000, the first year in which all five regions were represented (Fig. 6a). Density of EBP lobsters varied by region and month (2-way ANOVA, region F(4) = 7.4, P < 0.001; month F(5) = 13.6, P < 0.001; interaction F(17) = 2.7, P < 0.001). Post-hoc tests indicated that mean EBP density in 2000 was significantly higher in Massachusetts than in any other region (LSD test, P < 0.001 for PBME, CBME, and NH; P = 0.001 for SME) (Fig. 6a). In contrast, mean EBP density in Penobscot Bay was lower than that in any other region except southern Maine (LSD test, MA P < 0.001, CBME P = 0.002, NH P = 0.032, SME P = 0.260). Substrate temperature followed the same patterns (Fig. 6b), varying by region and month (2way ANOVA, region F(4) = 11.3, P < 0.001; month F(5) = 27.1, P < 0.001; interaction F(17) = 1.1, P < 0.001). Mean substrate temperature was higher in Massachusetts (14.4 ± 0.76°C) than in any other region and was lowest in Penobscot Bay (11.9 ± 0.50°C). To examine patterns of distribution within a large embayment, we sampled for juvenile lobsters at 16 intertidal sites in 1999 in Penobscot Bay (Fig. 7). No juvenile lobsters were detected at the 10 study sites in inner regions of Penobscot Bay, whereas lobsters were detected at the 6 sites in the outer portions of the bay (Fig. 7). Mean EBP lobster abundance in the inner bay (0 lobsters/m2) was significantly lower than that at outer sites (0.3 ± 0.03 lobsters/m2 ; t = –6.5, df = 783, P < 0.001) (Fig. 7). Our sample sizes of

8

0.20

6

0.15 0.10

4 5

6

8

7

9

10

Month

Fig. 4. Mean monthly density of EBP lobsters (lobsters/m2, closed circles) and substrate temperature (°C, open circles) as measured at 24 intertidal sites in the Gulf of Maine, 1997–2000 (n = 1417 EBP lobsters). Data from all sites and years are pooled by month. Vertical bars represent ± 1 SE.

Fig. 5. Highest monthly EBP density (lobsters/m2) at 24 intertidal sites in the Gulf of Maine, 1997–2000. Regions are arranged from north to south, and sites within regions are arranged from east to west. Regional abbreviations as in Fig. 3.


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Year 2000 a) 1.2

PBME CBME SME NH MA

EBP density (lobsters per m 2 )

1.0

0.8

0.6

0.4

0.2

0.0 5

6

7

8

9

10

Month

0.28 0.06

b)

0.20

22

Substrate Temperature ( C)

0.48

PBME CBME SME NH MA

20 18

0.56 0.03

16 14

Penobscot Bay

12

Maine

10 8 6 5

6

7

8

9

10

Month

Fig. 7. Detection of juvenile lobsters, Homarus americanus, at 16 intertidal sites studied in Penobscot Bay, Maine, 1999 (, lobsters detected; , lobsters not detected). Mean monthly densities of EBP lobsters (lobsters/m2) are noted at sites where lobsters were detected.

Fig. 6. Interregional comparison of (a) density of EBP lobsters and (b) substrate temperature in five regions around the Gulf of Maine, over the year-2000 sampling season. Data from year 2000 are pooled by month. Regional abbreviations as in Fig. 3. .7

1997

1998

1999

2000

Intertidal–subtidal comparison

.5

.4

.3

.2

.1

0.0 ct O p Se g Au y l Ju n Ju ay M ct O p Se g Au ly Ju n Ju y a M t c O p Se g Au ly Ju n Ju y a M t c O p Se g Au ly Ju n Ju y a M

To compare abundance estimates based on the JLMP with estimates based on subtidal suction sampling, we compared July/August densities at 13 subtidal sites in Penobscot Bay

EBP density (lobsters per m2)

.6

eastern and western sites (2 and 3) along the mouth of Penobscot Bay were too small to test statistically for differences, but intertidal densities of juvenile lobsters were generally higher in western than in eastern regions (Fig. 7). To test for interannual trends in EBP abundance, we used our longest time series, i.e., the four sites in Casco Bay that were monitored in all four years (Cundys Harbor, Little Harbor, Mackerel Cove, and Jaquish Island; Table 1). The general seasonal pattern of abundance was apparent in all four years (Fig. 8). No upward or downward interannual trend was detected in Casco Bay between 1997 and 2000 (Fig. 8), and years did not differ in mean annual EBP density (1-way ANOVA, F(3) = 2.1, P = 0.098).

Fig. 8. Time series of mean EBP lobster abundance (lobsters/m2) at four intertidal sites in Casco Bay studied 1997–2000.

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1.0

Subtidal EBP density (lobsters per m 2 )

Ð= 0.86 ð < 0.001 0.8

0.6

0.4

0.2

0.0

0.0

0.2

0.4

0.6

0.8

1.0

Intertidal EBP density (lobsters per m 2 )

Fig. 9. Density of EBP lobsters as measured at corresponding intertidal and subtidal sites in Penobscot Bay, Maine, July/August 1998–2000 (locations of sites in Table 2). Data from subtidal sites courtesy of Steneck and Wilson (2001).

(Table 2), where corresponding data existed between 1998 and 2000 (subtidal data published by Steneck and Wilson 2001 this issue). A strong positive correlation was apparent between densities of EBP lobsters in intertidal and subtidal sites (r = 0.86, P < 0.001)(Fig. 9). Discussion The JLMP takes advantage of the easy access to juvenile lobsters afforded by their lower-intertidal-zone habitat. Because the intertidal zone is the most landward margin of lobster distribution, there has been concern that abundance data gathered in the intertidal zone might not be representative of general patterns of abundance occurring subtidally. The strong correlation between abundances of EBP lobsters in the intertidal and subtidal zones (Fig. 9) shows that patterns of juvenile lobster abundances are similar in the two zones. The strong correlation of measured abundance, despite the different methods by which the data were gathered (venturi suction sampling in the subtidal zone and hand capture at low tide in the intertidal zone), strengthens the assertion (Cowan 1999) that the JLMP can serve as a low-cost, logistically simple method that can augment SCUBA-based sampling to provide greater temporal and spatial coverage of juvenile lobster abundance and distribution. Habitat use Juvenile lobsters were found in all five study regions around the Gulf of Maine. Rocky intertidal habitat appears to be preferred to eel-grass habitat by juvenile lobsters. Lobsters’ preference for rocks over eel-grass is consistent with data from subtidal coastal habitat studies by Wahle and Steneck (1991). Lobsters are most common in spatially complex habitats (Cooper and Uzmann 1980). Rocky habitats are

more complex than eel-grass beds and provide more protection from predators (Barshaw and Bryant-Rich 1988; Barshaw and Lavalli 1988). Rocks that provided shelter to juvenile lobsters in our study were similar in size and shape (39 × 28 × 15 cm) to those sampled at The Lobster Conservancy’s long-term study site, Lowell’s Cove in Casco Bay, Maine (30 × 30 × 10 cm) (Cowan 1999). On average, rocks were flattened horizontally (i.e., height was less than one-half the length or width), in contrast to the more spherical or cubical cobble, which has similar measures in all three dimensions and is usually the preferred subtidal habitat of juvenile lobsters (Wahle and Steneck 1991). In the turbulent intertidal zone, flattened rocks would be less likely to roll in response to wave action than would spherical or cubical rocks (Sousa 1984). Patterns of distribution and abundance Mean carapace length was significantly lower in Casco Bay than in other regions, suggesting a relatively higher proportion of small individuals. Casco Bay may therefore be a particularly important region for settlement and early growth. Alternatively this result may be biased by unequal sample size; considerably more data were collected in this region (Fig. 3a) than elsewhere, possibly leading to greater detection of smaller lobsters. As the JLMP continues, longterm sampling in all regions will increase sample size, allowing more thorough comparison of regions. Across all regions, a clear seasonal pattern of EBP abundance emerged, and high densities in mid-summer coincided with high temperatures. This result probably reflects temperature-dependent seasonal movements of individuals, similar to those exhibited by adults (Pezzack and Duggan 1986; Lawton and Lavalli 1995). In our 1-year interregional comparison, sites in Massachusetts had the highest abundance of EBP lobsters. Given the strong seasonal relationship between temperature and EBP abundance, we expected regions with higher temperatures to have higher juvenile abundance. This hypothesis was borne out by the interregional comparison of temperatures, because temperatures in Massachusetts were significantly higher than those in other regions. Within the Penobscot Bay region, juvenile lobsters were detected at intertidal sites in outer, but not inner, regions of the bay. A similar pattern was observed in a survey of 15 intertidal sites in Casco Bay, Maine, in which juvenile lobsters were found at sites exposed to open waters but not along the shoreline of inner bays and sounds (Cowan 1999). Similarly, Wahle (1993) studied subtidal lobster abundance in the Narragansett Bay estuary, Rhode Island, and found that abundances of both recently settled (5–10 mm CL) and larger lobsters were highest on the open coast, diminishing to zero in the upper bay. The mechanisms controlling these patterns are not known but probably reflect either unsuitable


conditions for settlement or reduced larval supply in inner bays (Wahle 1993). Physical factors that might influence suitability for settlement include salinity, temperature, oxygen levels, substrate type, wave action, ice scouring, and prevailing wind and water currents (Wahle 1993; Cowan 1999). Biological factors may include proximity to postlarval supply (Katz et al. 1994; Incze and Wahle 1991), shelter availability as refuge from predators (Wahle and Steneck 1992), and food availability (Lawton and Lavalli 1995). Postlarval supply might be smaller in the inner than the outer bays because currents carrying postlarvae might not reach inner regions of bays. Alternatively, postlarvae may settle out of the water column in outer regions before currents reach the inner regions. Ongoing research and modelling of larval distribution of Homarus americanus (see Incze and Naimie 2000) may help to distinguish between these competing hypotheses. Along the mouth of Penobscot Bay our sample sizes of eastern and western sites were too small to test for differences in juvenile abundance, but intertidal densities of juvenile lobsters tended to be higher in western than in eastern regions (Fig. 7). Similar patterns have been observed in the subtidal zone in Penobscot Bay (Steneck and Wilson 2001 this issue). The observed patterns of higher abundance of juvenile lobsters at western than at eastern sites in the bay support hypotheses that prevailing longshore currents, such as the Eastern Maine Coastal Current (Pettigrew et al. 1998), deliver proportionally more postlarvae to western than to eastern regions of Penobscot Bay (Steneck and Wilson 2001 this issue). Long-term sampling Long-term studies are critical for detecting processes important to long-lived species (Dye 1998), such as Homarus americanus. In addition, long-term baseline information can be used to explain the impact of unusual events (Dye 1998). To date, our longest volunteer-based time series in Casco Bay is too short for detecting trends in juvenile lobster abundance. The strength of the JLMP lies in its intended continuation over a period of decades. Over time, we will be able to test the hypothesis that recruitment to the fishery is influenced by the annual abundance of juvenile lobsters in prior years. This information will be useful for predictive fisheries models used to manage the resource. Benefits of volunteer-based research Our sampling program benefits from the participation of volunteers, which allows for cost-effective, long-term monitoring over a wide geographical range, at a time when long-term studies are difficult to fund. Volunteers, in turn, benefit from involvement in the program by receiving hands-on education about lobsters in particular and marine science in general.

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Involving citizen volunteers in long-term scientific research on crustaceans is a novel concept. This form of citizen participation in scientific research is a burgeoning phenomenon, at a time when many people are deeply concerned about degradation of the environment and feel compelled to make a difference (Youth 2000). The JLMP lends itself well to volunteer involvement for several reasons. The intertidal zone is easily accessible on foot, so no boats or specialized gear are required. Sampling occurs on monthly spring tides, which are predictable from tide tables and can therefore be scheduled in advance, a feature useful for people coming from various walks of life. Monitoring tools are inexpensive and easy to use, so they can be distributed to a large number of trained people. In comparison with conventional diver-based subtidal sampling, volunteer-based intertidal sampling is simpler and less expensive. We believe that the lobster fishery in the Gulf of Maine will benefit from improved scientific knowledge on abundance and distribution of juvenile lobsters, which can ultimately serve as input to predictive fisheries models, and from the increased environmental awareness and stewardship that is instilled in community volunteers who participate in the program. This volunteer-based research method may serve as a model for studying other crustaceans with near-shore juvenile stages. Acknowledgments Special thanks are extended to the many volunteers who have participated in the Juvenile Lobster Monitoring Program, without whom this research would not be feasible. We gratefully acknowledge financial support from the Community Fisheries Project of the Collaboration of Community Foundations for the Gulf of Maine, Darden Environmental Trust, Davis Conservation Foundation, Greater Piscataqua Community Foundation, Island Institute, Maine Community Foundation, Lobster Advisory Council of the Maine Department of Marine Resources, Maine/New Hampshire Sea Grant, National Fish and Wildlife Foundation, New England Grassroots Environment Fund, Up East Foundation, and individual donors. L. Hendrickson of NOAA Fisheries provided the GIS map in Fig. 1. The manuscript was improved by comments from section editor R. A. Wahle and two anonymous reviewers. References Able, K. W., Heck, K. L., Jr., Fahay, M. P., and Roman, C. T. (1988). Use of salt-marsh peat reefs by small juvenile lobsters on Cape Cod, Massachusetts. Estuaries 11, 83–86. ASMFC (Atlantic States Marine Fisheries Commission). (2000). Stock assessment report No. 00–01 (supplement) of the Atlantic States Marine Fisheries Commission: American Lobster stock assessment report for peer review. (Atlantic States Marine Fisheries Commission: Washington, D.C., USA.) 531 pp. Barshaw, D. E., and Bryant-Rich, D. R. (1988). A long-term study on the behavior and survival of early juvenile lobster, Homarus

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