Juvenile Lobsters at the New Outfall Site: Comparisons With an Inshore Population and Discussion of Potential Outfall Impacts on Lobster Populations

Transcription

1 Panel 1 (title) Juvenile Lobsters at the New Outfall Site: Comparisons With an Inshore Population and Discussion of Potential Outfall Impacts on Lobster Populations Kari L. Lavalli, Southwest Texas State University Roy K. Kropp, Battelle Duxbury Operations, Duxbury, Massachusetts Kenneth E. Keay, MWRA

2 Panel 2. Abstract In late May 1998, the Massachusetts Water Resources Authority (MWRA) was directed by the Outfall Monitoring Task Force to design and execute a study in the cobble-boulder habitats of the new outfall nearfield region to sample early benthic phase lobsters ( EBPs, 5 to 40 mm carapace length (CL)), particularly that of new recruits ( young-of-theyear, <12 mm CL) and yearling lobsters (shelter-restricted, <20 mm CL). Both of these life history phases are thought to be relatively nonmobile, obligate shelter-dwellers. MWRA was also required to determine if the numbers of these life history stages were comparable to those of nearby inshore habitats. This mandate resulted from serious concerns about the effects that the new outfall might have on juvenile lobsters and thus the future of the economically important lobster fishery. MWRA responded to this mandate by proposing a survey plan that used examination of videotapes from a remotelyoperated vehicle survey, previous data on lobster density from hard bottom surveys, and used a mathematical calculation to determine suitable locations for sampling and an appropriate sample size for the collection of species occurring only rarely in a region. These tactics were designed to maximize the chances of locating young-of-the-year and shelterrestricted lobsters at the outfall vicinity. In early September 1998, density sampling was undertaken by the foremost experts in airlifting for lobsters underwater at both the vicinity of the outfall and two nearby inshore stations. The data collected showed significantly lower densities of young-of-the-year, yearling lobsters, and larger EBP lobsters at the outfall compared to the inshore sites. Measures of the proportion of non-zero observations (which is another measure of frequency) for each size class also showed significantly fewer non-zero observations at the outfall. Taken together, these data demonstrate that while the cobble habitat at the vicinity of the outfall is suitable for settlement, it does not represent a major settlement site and thus there is no indication that the outfall will have any appreciable impact on these life stages of the American lobster.

3 Panel 3. Background In 1997, concern was raised over the potential impact of discharges from the new MWRA outfall in Massachusetts Bay on the recruitment of postlarval lobsters to the area. The Outfall Monitoring Task Force (OMTF) designated a Lobster Larvae Focus Group (LLFG) to address the issue. In October 1997, the LLFG met and recommended that MWRA evaluate the habitat value of the new outfall area. In November 1997, MWRA authorized a special task to review the biology of the lobster, Homarus americanus, in Boston Harbor and Massachusetts Bay to evaluate the potential risk of the outfall to lobster populations. The report that resulted from this task concluded that the new MWRA outfall would pose no significant potential risk to the pelagic larval and benthic juvenile lifestages of the American lobster. In April 1998, Drs. K. Lavalli and D. Cowan independently prepared a report on the issue for the OMTF focus group. This report concluded that the new MWRA outfall would not have an effect on lobster larvae. In May 1998, the OMTF directed MWRA to conduct a suction-sampling survey to sample early benthic phase (EBP) lobsters in the vicinity of the new outfall and to compare abundances there to those in nearby coastal waters. MWRA authorized the design for such a survey. The proposed design was submitted to the OMTF and other parties, including regional experts in lobster population ecology, for review. No significant comments on the design were received and the survey was conducted on September 8-9, This poster summarizes the results of that survey.

4 Panel 4. Background (cont.) Notes on Lobster Larval Biology In summer, females move to warmer shallow waters to release larvae to the plankton Three planktonic larval stages develop rapidly (2 8 weeks depending on water temperature) and subsequently molt to a fourth, or postlarval stage. The postlarval stage lasts about 11 days (again depending on temperature). Settlement is not random. Postlarvae settle primarily in cobble, rock on sand, or peat reef habitats. Terminology for recently settled lobsters varies considerably. For this study we consider Young-of-the-Year (YOY) lobsters as those having a carapace length (CL) < 12 mm; Yearlings as those having a CL > 12 mm, but < 20 mm; and early benthic phase (EBP) lobsters as those having a CL < 40 mm. YOY are shelter restricted, that is, they settle in an area and do not make significant movements Yearlings have passed through one winter and make limited movements near its shelter EBPs larger than 20 mm CL are also known as vagile juveniles and often move to new habitats. See figure of larval life history stages File: lifecycle.pdf

5 Panel 5. Background (cont.) Factors of Lobster Larval Biology Important to the Outfall Issue Postlarval settlement occurs in late summer, July through September, but varies by region. Most postlarval settlement occurs in shallow waters, primarily those shallower than 10 m. Cobble habitat is probably the prime site for settlement although other habitats serve as recruitment sites. However, not all cobble habitat is settled. Most postlarvae will not cross a strong thermal gradient (~ 5 ºC), thus a thermocline will inhibit settlement. Postlarval growth and survival is greatly reduced at water temperatures below 10 ºC. At temperatures below 5 ºC, growth may be totally inhibited and grow-out time greatly extended, thus prolonging susceptibility to predation.

6 Panel 6. Background (cont.) Conditions at the Outfall Site Water depth is about 27 m. There is considerable cobble habitat. A strong thermocline is in place from about May to October. The temperature difference across the thermocline is about 10 ºC from June to mid-september. Nearfield bottom water temperatures, taken at stations N04, N07, N16, N20, are below 10 ºC most of the time o 10 Temperature (C) Jan- 92 Apr- 92 Jul- 92 Oct- 92 Jan- 93 Apr- 93 Jul- 93 Oct- 93 Jan- 94 Apr- 94 Jul- 94 Oct- 94 Jan- 95 Apr- 95 Jul- 95 Oct- 95 Jan- 96 Apr- 96 Jul- 96 Oct- 96 Jan- 97 Apr- 97 Jul- 97 Oct- 97 Jan- 98 Apr- 98 Jul- 98 Oct- 98

7 Panel 7. Survey Objectives Consideration of larval biology and outfall conditions suggested that despite the cobble habitat present at the Outfall site, postlarval settlement was expected to be very low because of the depth of the habitat and the presence of a strong thermocline during the peak settlement period. Any postlarvae that might settle would experience low growth and survival rates because of the consistently low temperatures at the outfall. Thus the outfall area was not expected to represent a significant settlement site for American lobsters. However, the OMTF directed MWRA to conduct a survey to evaluate the outfall area as a settlement site. Following the OMTF directive, the objectives of the survey were to Determine if EBP lobsters were present at the new MWRA Outfall site in Massachusetts Bay Compare the numbers of EBP lobsters at the Outfall site with those at a nearby Inshore site

8 Panel 8. Survey Approach Sampling Locations Outfall Site: We examined extensive videotapes obtained by a 1994 remotely-operated vehicle (ROV) survey of the Outfall area (Coats and Campbell 1994, Coats et al. 1995) to identify the cobble-boulder habitats most likely to harbor juvenile lobsters, and to identify the subsections of transects where these habitats lay. The best lobster habitats were typically sand substrates that had a cover of 26 to >75% of rocks (6 in to 3 ft in diameter) and boulders (3 to 9 ft in diameter) colonized by hydroids or covered with detritus. The best subsections identified typically had more than 75% of rocks and boulders and yet were still capable of being sampled by airlifts and quadrats (i.e., the boulders weren't of a size that would be impossible for divers to move). These habitats provide a structurally complex environment with rich food sources for lobsters. We also noted the presence of smaller juveniles (vagile or adolescent phase lobsters) on the videotapes and used data on lobster presence from Coats and Campbell (1994) to help determine the final sampling locations. Depth was also considered because the sampling was to be conducted by divers using Nitrox diving techniques. To stay within safe-diving limits, sampling locations were chosen that were < 29 m in depth (mean low water). Inshore Site: We used lobster density data collected by the Massachusetts Department of Marine Fisheries (MADMF) to determine an appropriate inshore site to sample. MADMF samples tow areas relatively close to the Outfall, Boston Harbor and Salem Harbor. However, only one year s data were available for Boston Harbor, whereas several year s data were available for Salem Harbor (Coney Island and Bakers Island). Therefore, we chose Salem Harbor as the Inshore site. MADMF data from Salem Harbor would allow us to verify that the 1998 data collected during this study reasonably depicted lobster numbers from the Harbor.

9 Panel 9. Survey Approach (cont.) The sites selected for the September 1998 suction-sampling survey were Outfall: Transect T-1, Subsection 4, Transect T-4, Subsection 9, and Transect T-6, Subsection 6. Inshore: Salem Harbor: off Coney Island, and off Bakers Island Outfall and Salem Map Salem Harbor Map See Figure 1a (file: outfall.pdf) See Figure 1b (file: salemharbor.pdf)

10 Panel 10. Survey Approach (cont.) Sample Size Previous sample sizes for lobster suction-sampling studies has varied, ranging from 16 Η 0.25 m 2 quadrats (total area sampled = 4 m 2 ) to 12 Η 0.5 m 2 quadrats (total area sampled = 6 m 2 ). Examples of the ratio of area sampled to total area of the study are 20 m 2 sampled along 13 km of coast (coastal Maine) and 33 m 2 sampled in an area of about 265 km 2 (Narragansett Bay). We suspected that EBP lobsters would be rare at best near the Outfall, therefore we used a conservative approach based on a technique proposed by Green and Young (1993) that allowed us to calculate a minimum sample size to detect rare EBP lobsters. Using this technique, we calculated that 32 Η 0.5 m 2 quadrats would be necessary to yield a 95% probability of finding rare EBP lobsters at the Outfall. Conservatively, we assigned 36 as the number of quadrats to be sampled, 12 at each Outfall area. To provide an equal sample size for the Inshore site, 18 quadrats were to be sampled at each Salem Harbor location (Bakers Island, Coney Island). The sampling effort (18 m 2 sampled in the Outfall area of ~18 km 2, and the 36 quadrats per site) used in this study was greater than any previous study to assess lobster populations Sampling Time Lobster postlarvae typically settle during the summer from July through September Settlement occurs earlier in southern (mid-august Rhode Island) than in northern New England (September Maine) Data from Nahant Massachusetts (collected by Lavalli) indicate that YOY were not common until after mid-august Therefore sampling in September was most likely to occur after the peak settlement of YOY lobsters

11 Panel 11. Survey Approach (cont.) Sampling Methods The survey was conducted September 8, 9, 1998 onboard the R/V Gulf Challenger (University of New Hampshire) Divers from Dr. Robert Steneck s lab (University of Maine) and Dr. Richard Wahle s lab (Bigelow Labs) performed all of the sampling. These two labs have pioneered larval lobster studies in the region. At each location, divers placed 0.5 m 2 quadrats haphazardly on the bottom in areas that could be sampled adequately by using an air-lift suction sampler Prior to suction sampling, all mobile megafauna, including any larger lobsters that would not be captured by the sampler, were removed from the quadrat an placed in a mesh bag Suction sampling proceeded as one diver carefully removed rocks and boulders while another diver operated the air-lift sampler. Samples were sorted by hand on board the Gulf Challenger; all lobsters were removed, the carapace length was measured to the nearest 0.1 mm, the gender of each lobster was determined. See photo of onboard sorting operations (file: hunt sorts.pdf) See photo of Young of the Year (YOY) lobster (file: YOY from inshore.pdf)

12 Panel 12. Results Site Characteristics Site characteristics for locations sampled on the September 1998 survey. Location Coordinates Depth (below MLW) Water Tempera ture (at bottom) Substrate Algae Types Present Other Species Present Outfall T6, S 'N 'W 89 ft (27 m) 40 F (~4 C) Rocks 2 >50 cm diameter Coralline Ophiuroids, small sea stars, decapod shrimp, one axiid shrimp, amphipods, polychaetes Outfall T4, S 'N 'W 80 ft (24 m) 40 F (~4 C) Rocks 2 >50 cm diameter Coralline Ophiuroids, small sea stars, decapod shrimp, one axiid shrimp, amphipods, polychaetes Outfall T1, S 'N 'W ft (25 27 m) 37 F (~3 C) Rocks 2 50 cm diameter Coralline; Some Kelp Ophiuroids, decapod shrimp, sea stars polychaetes, sea urchins, amphipods Bakers Island 'N 'W 27 ft (8 m) Not taken Rocks 2 >50 cm diameter Coralline; Urchin Grazed Cancrid crabs (Cancer irroratus, C. borealis); green crabs (Carcinus maenas), snails (L. littoralis); mussels (M. edulis), nereid and/or nephtyid polychaetes Coney Island 'N 'W 16 ft (5 m) 60 F (~16 C) Rocks 2 50 cm diameter Kelp; Red Foliose Species Cancrid crabs (Cancer irroratus, C. borealis); green crabs (Carcinus maenas), snails (L. littoralis); mussels (M. edulis), nereid and/or nephtyid polychaetes

13 Panel 13. Results (cont.) Lobster Numbers 25 lobsters were collected at the Inshore site (Salem Harbor) 2 lobsters were collected at the new Outfall site Characteristics of lobsters collected on the September 1998 survey. Site Bakers Island Coney Island Outfall Site Age Class Size (mm CL) Gender Yearling (Shelter-Restricted and Emergent Juvenile) Undetermined Undetermined Male Vagile Juvenile 22.4 Female 25.5 Male 30.0 Not Recorded 30.4 Female 30.5 Female Adult Male YOY (Shelter-Restricted Juvenile) 7.8 Undetermined 9.3 Undetermined 9.9 Undetermined 10.8 Undetermined Yearling 13.3 Undetermined (Shelter-Restricted and Emergent Juvenile) 19.7 Vagile Juvenile Male Male Male Female Male Male Female Male Adolescent Female Female Male YOY (Shelter-Restricted Juvenile 8.6 Undetermined Vagile Juvenile 35.0 Male

14 Panel 14. Results (cont.) The differences between Outfall and Inshore sites sampled in September 1998 were statistically significant for all categories of lobsters Lobster Category Outfall Site (#) Inshore Site (#) Kruskal-Wallis p YOY/ Yearling (< 20 mm CL) 1 9 < EPB (< 40 mm CL) 2 21 < All Lobsters 2 25 <0.001

15 Panel 15. Results (cont.) The proportion of non-zero quadrats differed significantly between Outfall and Inshore sites (G-test) For YOY/yearling lobsters (p < ) Region Quadrats (#) Zero (#) Non-Zero (#) % Non-Zero Outfall Site Inshore Site * For EPB lobsters (p < Region Quadrats (#) Zero (#) Non-Zero (#) % Non-Zero Outfall Site Inshore Site * For all lobsters (p < Region Quadrats (#) Zero (#) Non-Zero (#) % Non-Zero Outfall Site Inshore Site *

16 Panel 16. Results (cont.) The 1998 Inshore results did not differ from previous MADMF results (p > 0.05; Kruskal-Wallis, nonparametric multiple comparison tests) NOTE: In this case, YOY is defined as <12 mm CL to make the MWRA survey data directly comparable to MADMF s data. Site Age Class MADMF 1995 MADMF 1997 MADMF 1998 MWRA 1998 Bakers Island Coney Island YOY (< 12 mm CL) EBP (5 40 mm CL) YOY (< 12 mm CL) EBP (5 40 mm CL) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Therefore the 1998 survey data reasonably reflect larval lobster numbers in Salem Harbor.

17 Panel 17. Conclusions Lobster postlarvae may settle near the new outfall, but postlarvae are rare. However, the Outfall site is NOT a significant site lobster postlarval settlement compared to an Inshore site. Any postlarvae that do settle near the outfall would have severely retarded growth rates and low survival rates because of the low near-bottom water temperatures present there most of the time. The new outfall will not have a significant adverse impact on the settlement of postlarval lobsters. Acknowledgements We gratefully acknowledge the extraordinary efforts put into the collection of this data by the scientific diving crews of Dr. Robert Steneck and Mr. Carl Wilson (University of Maine, Walpole, Maine) and Dr. Richard Wahle (Bigelow Laboratory for Ocean Sciences, Boothbay Harbor, Maine). We also thank Drs. Wahle, Steneck, and Bruce Estrella (MADMF) for sharing their unpublished benthic sampling data to make the necessary comparisons with densities obtained in this survey and for their criticisms and comments on both the survey plan and the relevance of the data. Others who aided in the design of this study include Vin Malkoski, dive officer from MADMF; Dr. Doug Coats from Marine Research Specialists, Ventura, California; and Edward Maney, President of AAUS.