Combining Data from Space With Local Knowledge to Sustain an Important Fishery

Information on the ecology of Penobscot Bay has formed the basis for an intensive analysis of the bay's lobster fishery. By combining data from satellites and field sampling with the local knowledge of fishermen, the project has created a broader understanding of the fishery that includes much of its ecological context. Understanding how the physical properties of the Penobscot Bay ecosystem affect the bay's biology, including its lobster population, will ultimately help to sustain the fishery upon which many in the area depend.

Penobscot Bay Hydrographic Studies Yield Surprising Results

Initial studies designed to provide an ecological characterization of Penobscot Bay indicate that it is not a typical estuary as had been anticipated. Despite significant freshwater input from the Penobscot River flowing into the bay at its head, circulation in the bay is dominated by inflows from the currents of the neighboring Gulf of Maine. According to project scientist Neal Pettigrew of the University of Maine, the movement of water westward along the shore of the eastern Gulf of Maine-the Eastern Maine Coastal Current-branches off as it nears Penobscot Bay, sending 10 times the volume of water into the bay as the river does. During the summer, surface currents appear to flow into the western bay and out the eastern bay creating a gyre around the large, mid-bay islands of North Haven and Vinalhaven.

As a result of this finding, project scientists hypothesize that the bay's thriving lobster fishery may depend upon larvae imported into the bay during the summer through interactions with the Eastern Maine Coastal Current. These scientists have embarked upon a multi-disciplinary study to characterize the ways in which physical and biological processes affect the various life stages of lobsters - from egg to harvestable adult. The study includes traditional oceanographic techniques, as well as data collected with the help of fishermen and shoreside volunteers. The ultimate goal is to build a predictive model of lobster recruitment in Penobscot Bay.

Such a model will support a remarkable new experiment in fisheries management in which government, scientists, and fishermen work together to “co-manage” Maine's lobster fishery. The goal is for these co-managers to be able to use readily-available satellite data to help predict the scale of future harvests in the bay and to manage fishing effort accordingly.

An on-board round-table discussion of how fishery managers can apply remote sensing data.

Lobster Study Starts With Brood Stock

If larvae are coming from outside Penobscot Bay, what is their source? By piecing together what is known about the development of lobsters, project scientists have hypothesized that larvae entering Penobscot Bay most likely originate from brood stock found “upstream” or farther to the east. How far? Larval lobsters are planktonic for a month or more after hatching in early summer. Although they can swim, their movement is largely a function of the currents in which they become entrained.

The rate of development of the larvae is in part a function of water temperature. Therefore, using Penobscot Bay as the end point and working backwards, scientists have been able to use information on development rates, water temperature, and currents to estimate the potential locations of the brood stock from which the larvae originated. In 1999, project scientists Bob Steneck of the University of Maine and Carl Wilson of the Island Institute, used a remotely-operated vehicle to search for the female lobsters, or brood stock, that might contribute to at least some of Penobscot Bay's lobster larvae. Results of their survey indicate that brood stock may occur in significant numbers close to shore.

After hatching, lobsters go through four stages of development before settling to the bottom. In the last stage of development, before settlement, lobsters are known as postlarvae; they have the same general shape as adult lobsters but are less than an inch long. While the earlier larval stages can be found at a variety of depths in the water column, postlarvae tend to rise to the surface layers of the ocean where winds might have a particularly strong effect on their movement. The research of project scientist Lew Incze, from the Bigelow Laboratory for Ocean Sciences, focuses on tracking larvae as they enter Penobscot Bay. By sampling the surface waters of the bay and its approaches, and collaborating with project oceanographers Neal Pettigrew (currents) and Andrew Thomas (satellite data) and modeler Huijie Xue (model), Incze has been able to characterize postlarval populations in the bay and understand their relationship to currents and winds.

Incze began his two-year study in 1999. First stage larvae were present in the surface layer south of the bay and in the western bay. Postlarvae were rare throughout the entire sampling period (late June through early September); those found were mostly in waters south of the bay. These findings are consistent with the view that the source of most of the larvae settling in the bay is the coastal waters outside the bay and that most transport into the bay is via the west channel.

Project scientist Eric Annis, a graduate student at the University of Maine, surveyed the area of the Eastern Maine Coastal Current in early August 1999 for larval lobsters. Eric found mostly early stage larvae inshore; postlarvae were found inshore only west of Penobscot Bay, and the remainder occurred offshore of the Eastern Maine Coastal Current. The offshore distribution is consistent with earlier modeling results, which suggest transport from distant sources to the east, but why such low numbers inshore? One possible explanation is that the Eastern Maine Coastal Current carried most larvae/postlarvae westward, where they may have accumulated in the stratified frontal region west of Penobscot Bay. Thus, the interaction of the Eastern Maine Coastal Current with nearshore waters may have positive or negative effects on recruitment of larvae, depending on the particular locale and circumstances. We know from prior results and landings that recruitment is sometimes good in western Penobscot Bay and its approaches; this does not seem to have been true in 1999. The mechanisms contributing to variable recruitment are a focus of the field work and modeling.

Satellite Role in Tracking Larvae Explored

Currents can be characterized by careful analysis of satellite data on sea surface temperature. Project scientist Andrew Thomas of the University of Maine has analyzed a twelve-year archive of sea surface temperature images dating from 1985-1996. His analysis reveals that patterns of sea surface temperature associated with the Eastern Maine Coastal Current are stable over the long-term. Within Penobscot Bay, the western bay appears to be consistently warmer than the eastern bay during spring, summer, and fall over the twelve-year period. In addition, a surface temperature gradient across the entrance to the western Bay during the summer is relatively stable over time, with warmer water inside and colder water outside the bay.

Analysis indicates that, in addition to seasonal temperature changes, there are also relatively strong (up to 1.5 C) shifts in surface temperature from year-to-year. The spatial patterns of these shifts, known as anomalies, have been quantified. Anomalies within Penobscot Bay show a pattern of correlation with anomalies further upstream in the Eastern Maine Coastal Current. Project scientists hypothesize that shifts in the pattern of sea surface temperature from year-to-year, as indicated by anomalies, may strongly influence the number of larvae entering the bay. The position of fronts in the Eastern Maine Coastal Current, relative to the mouth of the bay, may be critical to determining how many larvae reach the bay's nursery grounds in any given year. In 1999, Thomas began work with other project scientists to compare patterns of sea surface temperature over time with patterns of lobster settlement for the same period. Initial results of this analysis indicate that it may contribute to the development of a predictive model.

Larvae Seek Preferred Habitat

Having been carried by currents for a month or more after hatching, the tiny lobsters begin to settle to the bottom searching for their preferred habitat. Most of the settled lobsters are found in cobble areas in shallow water. Project scientists Joe Kelley of the University of Maine and Steve Dickson of the Maine Department of Conservation have extensively mapped the sediments of Penobscot Bay. Analyzing these data in relation to the depth, project scientist Chris Brehme of the Island Institute has identified those areas of the bay that are potential settling habitat for postlarval lobsters. One question is: how many postlarvae find these habitats?

Steneck and Wilson have initiated a two-phase sampling program. During the summer months, divers visually survey juvenile lobsters within quadrats. In early fall, divers use suction samplers to quantify the abundance and distribution of newly settled lobsters in areas of potential settling habitat. High-density sampling was conducted in 1999 to characterize patterns of settlement throughout the bay. Analysis of the data revealed a decline in lobster settlement in the Penobscot Bay region compared to the previous year. Patterns of settlement are consistent with previous samplings; highest settlement continues to occur in the western mouth of Penobscot Bay but there are distinct settlement cold spots or shadows. In addition, there is a very clear north/south and east/west pattern. These strong gradients of abundance make Penobscot Bay, a good laboratory for understanding lobster settlement dynamics. The northeast corner of the bay is a lobster desert; settlement is virtually absent. Adolescent phase lobsters move into this region, but their population densities are low as are catch rates of sublegal lobsters.

In a parallel effort, project scientists Sara Ellis and Diane Cowan of The Lobster Conservancy, are analyzing lobster settlement by surveying the lower intertidal zone for young lobsters. In 1999, the distribution of early benthic phase lobsters found in the intertidal zone correlated with Steneck and Wilson's data on the distribution of this stage subtidally. The Conservancy's novel methodology complements diver surveys with an activity than can be carried out by volunteers, is inexpensive, and provides educational opportunities.

Examining the rocky shore for lobster settlement.

Project scientists Incze and Rick Wahle of the Bigelow Laboratory have analyzed settlement of postlarvae at several sites in the region over the last 10 years. The data indicate an overall pattern: settlement may be rebounding after a decline that started in 1995. The pattern indicates a geographic lag effect; changes in patterns of settlement are reflected first at stations to the west. Penobscot Bay, at the eastern end of the stations, is still showing a decline in settlement. Project scientists will be watching to see if this pattern is reversed in the coming years as would be expected from the regional time series data.

Understanding Lobster Growth and Development

After settlement, it takes approximately seven years for young lobsters to reach harvestable size. Surprisingly, there is apparently little predation on lobsters once they have settled and found suitable habitat. This may be due, in part, to the fact that inshore waters no longer harbor stocks of groundfish capable of consuming lobsters. In any case, earlier work by Incze and colleagues Rick Wahle (Bigelow Laboratory) and Stan Cobb (University of Rhode Island) demonstrated that there is a close relationship between the abundance of postlarvae and the abundance of settlers. Also, there is some indication that the abundance of settled lobsters is closely related to the size of the population even several years later.

Two project activities examine what happens to the lobster population in the long interval between settlement and harvest. In the first activity, Bob Steneck of the University of Maine and Carl Wilson of the Island Institute are using lobster boats as platforms for assessing the size and distribution of the bay's lobster population. By analyzing the contents of a fisherman's hauls and relating them to the area fished, Steneck and Wilson are able to collect information on the relative number of lobsters in each of several size classes. In addition, information is collected on the numbers of each sex, egg-bearing females, v-notched females (egg-bearing females are v-notched in one of their tail flippers by fishermen to protect them from harvest even when they are not bearing eggs), and one-claw lobsters (“culls”). In 1999, abundance of sublegal lobsters correlated fairly well throughout the bay with adolescent phase lobsters suctioned-sampled on the bottom, indicating good linkages between patterns of settlement and harvest.

In the second study, project scientist Rick Wahle is looking at variability in the growth, mortality, and movement of young lobsters. In the summer of 1999, Wahle began a Sea Grant-supported two-year project that, in part, complements the Penobscot Bay project. This study spans sites in Rhode Island, New Hampshire and Maine. The aim of this project is to better understand the ecological processes - such as mortality, movements, and growth - during the critical phase when lobsters emerge from nursery grounds, but before they enter the fishery.

In this study, multiple mark-recapture experiments are being conducted by divers. Lobsters are tagged and recaptured in defined 30-meters by 30-meters areas of cobble nursery beds at each site. Comparing the exchange of lobsters between four equal quadrants of the square to overall losses from the entire large square permit the researcher to distinguish and estimate losses due to emigration and mortality. Video surveys of habitat and fish predator abundance by divers aid in evaluating the relationship between habitat quality and lobster movements and mortality. Reports of tagged lobsters from fishermen contribute valuable additional data on movements beyond the study areas. Preliminary results from the first year of the study indicate that emigration due to habitat effects is a stronger factor than mortality in turnover of settling lobsters.

Using Satellite Data For a Predictive Model

There is remarkable confluence of results among the research studies outlined above. Collectively, they establish a consistent picture of the dynamics of Penobscot Bay's lobster population: the potential sources of larvae, factors affecting their transport to and settlement in the bay, and patterns in growth and development as they mature and recruit into the fishery. This information will allow project scientists to create a model that will help the fishery's co-managers (fishermen and state regulators) understand and sustain the bay's lobster fishery.

As the basis for such an effort, project scientist Huijie Xue of the University of Maine has created an operational, numerical model of Penobscot Bay circulation. The model simulates those processes that drive the physical properties of the bay, including surface wind, surface heat, and freshwater fluxes, river discharge, and both tidal and subtidal inflow from the larger Gulf of Maine. The model simulates current velocity, temperature, and salinity in three dimensions, at different times, and during special events, such as high winds.

Indications of a strong fit between biological observations and their physical context, now described by Xue's model, indicate good prospects for a predictive model of recruitment into Penobscot Bay's lobster fishery. By running the model every year with inputs from up-to-date satellite images, and by monitoring data from fishermen, scientists, and volunteers, co-managers will have the information they need to ensure a healthy future, not only for the lobsters of Penobscot Bay, but for the coastal economy that depends upon them.

Related Web Links

Intertidal Lobster Monitoring Program

Post-larval Lobster Distribution in Penobscot Bay in Relation to Hydrography, Circulation and Remote Sensing Information

Island Institute - This Web site provides information about the State of Maine and the Penobscot Bay initiative.

NOAA National Environmental Satellite, Data, and Information Service (NESDIS) - This page provides information about NESDIS offices and provides links to national data centers.

The Lobster Conservancy - This Web site provides extensive information on lobster biology.

Bigelow Laboratory - This Web site provides information about scientific research and educational resources that are related to marine science.