Ecological implications of a northern range expansion of black sea bass, Centropristis striata

Transcription

1 Ecological implications of a northern range expansion of black sea bass, Centropristis striata Northeastern University 430 Nahant Rd Nahant, MA mcmahan.m@neu.edu Jonathan H. Grabowski Northeastern University Associate Professor 430 Nahant Rd Nahant, MA j.grabowski@neu.edu x 337 ABSTRACT The coastal waters of the Northwest Atlantic contain one of the steepest temperature gradients in the world. In the past, this significant variation in water temperature has prevented temperate species from shifting northward. However, the general rise in temperature over the past two decades, as well as events such as the 2012 warm water temperature anomaly that ranged from Cape Hatteras, NC to Labrador Canada, have resulted in more temperate species entering the Gulf of Maine. For example, black sea bass, Centropristis striata, historically ranged from the Gulf of Mexico to Cape Cod, but in recent years it has been reported as far north as midcoast Maine. While this apparent northern range expansion is likely due to increasing water temperatures in the Gulf of Maine, the ecological impacts of black sea bass range expansion are uncertain. I am proposing to quantify sea bass range expansion and its impact on benthic community structure throughout the coastal waters of the Gulf of Maine. This project will provide a framework for how to monitor the emergence of new species and their impacts on coastal ecosystems. As regions such as the Gulf of Maine undergo climate-induced warming and shift to an ecosystem approach to fisheries management, a greater understanding is needed of how range expansions and contractions will impact food web dynamics and fisheries productivity of native species.

2 Introduction The warming of the world s oceans has begun to restructure marine communities and impact system productivity (Mooney et al. 2009). This climate-induced environmental forcing is occurring at a rapid rate, increasing the need for efficient assessment of climate change impacts to local ecosystems. One of the consequences of warming ocean temperatures is that species ranges often expand or contract to avoid temperature stress. The Gulf of Maine provides an excellent platform for studying range shifts because of the steep temperature gradient that exists in the Northwest Atlantic. In the past, this temperature gradient impeded many species from moving northward. However, warming trends over the past several decades, as well as the 2012 warm water temperature anomaly that ranged from Cape Hatteras, NC to Labrador Canada (Mills et al. 2013), have resulted in a northern shift of many temperate species. The black sea bass (Centropristis striata) ranges from the Gulf of Maine (GOM) to the Gulf of Mexico and supports economically valuable commercial and recreational fisheries (Moser and Shepherd 2009). The northern stock ranges from Cape Hatteras, North Carolina to Massachusetts and is part of a single fishery management unit (Shepherd 2008). Historically, black sea bass were thought to be rare in the waters north of Cape Cod (DeWitt et al 1981; Drohan et al. 2007). In recent years, however, there is evidence from bycatch of sea bass by commercial fishermen, as well as direct observation via SCUBA, that black sea bass have expanded their range as far north as midcoast Maine (Marissa McMahan unpublished data). Moreover, McMahan (unpublished data) has collected over 50 sea bass samples from fishermen in the midcoast Maine area and has directly observed sea bass via SCUBA and video as far north as Muscungus Bay, Maine. Typically, black sea bass are found in waters ranging from C (Steimle et al. 1999). The average annual sea surface temperature in the GOM is roughly 7 C (Shearman and Lentz 2010); however, over the past several decades, ocean temperatures have significantly increased (P < 0.05) in nearshore coastal Maine (Maine Department of Marine Resources 2013, Mills et al. 2013). The average temperature in midcoast Maine from June through September, when sea bass are typically found in nearshore areas, was 14.9 C in the 1990s and 16.9 C in the 2000s, an increase of 2 C (Maine Department of Marine Resources 2013). This observed warming of sea water temperatures is likely enabling the northern range expansion of black sea bass. Currently, studies and monitoring data on black sea bass in the GOM are limited. Both the Northeast Fisheries Science Center and the state-led inshore bottom trawl surveys cover much of the GOM and provide a measure of relative abundance of each fish species that is caught. However, black sea bass are typically found in structurally complex habitat, such as oyster and rocky reefs (Peterson et al. 2003; Drohan et al. 2007), that is not conducive to sampling with otter trawls. Therefore, these surveys are not considered an accurate measure of sea bass abundance (Shepherd 2008). In addition, black sea bass are not managed north of Massachusetts, despite the increase in both sightings and catch in the northern GOM in recent years. Efforts to describe their abundance and distribution shifts in coastal waters of the GOM are a necessary first step in understanding the ecological impacts and economic implications of their range expansion. Black sea bass use similar habitats with numerous species within the GOM such as the American lobster, which represents the second most valuable fishery in the United States (Fisheries of the United States 2011). While black sea bass south of Cape Cod have been found to consume a diet largely comprised (>50%) of decapods (Garrison and Link 2000), it is currently uncertain the degree to which northward expansion of black sea bass is affecting lobster population dynamics. Therefore, efforts to quantify the distribution, abundance, and ecology of black sea bass throughout the coastal waters of the GOM will broaden our understanding of their impacts on food web dynamics and fisheries productivity as they become established in their newly expanded range. Objectives The purpose of this project is to quantify the degree of black sea bass range expansion and assess the impact of this expansion on the community structure and associated fisheries of the coastal waters of the

3 GOM. Observing how distribution and feeding ecology vary across a north-south gradient will provide evidence of how rapidly the range expansion is occurring. It will also reveal how sea bass are impacting benthic communities in coastal waters of the GOM. Greater understanding of black sea bass range expansion will lead to more informed stock assessments and management of the northern stock (NAFO Div. 5YZ, 6ABC). Overall, this project will provide a framework for future monitoring of emergent species in the GOM. Objective 1: To quantify the distribution, relative abundance, and size frequency of black sea bass in its native and newly expanded range. Hypothesis 1: Black sea bass will be least abundant in the northern region and most abundant in the southern region. There will be a higher ratio of adults to juveniles in the northern region where water temperatures are colder. Objective 2: To observe black sea bass behavior and determine the impact of sea bass range expansion on benthic communities and native fisheries. Hypothesis 2A: Black sea bass will exhibit high site fidelity and aggressive behavior towards local species with overlapping niches. Hypothesis 2B: The prevalence of lobsters in the diet of black sea bass will increase from Rhode Island to Maine. Furthermore, black sea bass will negatively impact lobster populations in coastal waters of the GOM. Methods Objective 1: To quantify the distribution, relative abundance, and size frequency of black sea bass in its native and newly expanded range. SCUBA surveys: Black sea bass exhibit strong site fidelity and are found in relatively shallow coastal waters (Drohan et al. 2007), which makes them an ideal species for observational surveys via SCUBA. To test hypothesis 1, underwater surveys of black sea bass will be conducted by SCUBA* in two regions (midcoast Maine and Massachusetts) within the GOM and one region (Rhode Island) south of Cape Cod (Fig. 1). The Rhode Island site is within the historic northern range limit of black sea bass, the Massachusetts site is within the more recently expanded northern range of black sea bass, and the midcoast Maine site represents the northern extent of where there have been reports of black sea bass sightings. Within each region, three sites with rocky bottom habitat will be sampled. Surveys at each site will consist of three consecutive transects measuring 30 x 4 meters at roughly 10 meters depth. Each site will be sampled monthly between May and October. Black sea bass abundance (i.e., number of individuals) and size of individuals (total length to the nearest cm) will be recorded in each transect, and abundance will be used to calculate black sea bass density (#/m 2 ) at each site during each season. Temperature and salinity will be recorded prior to the start of the dive, and depth will be recorded within each transect. *All diving conducted for research purposes will be under the auspices of Northeastern University s AAUS dive program. Objective 2: To observe black sea bass behavior and determine the impact of sea bass range expansion on benthic communities and native fisheries. Video analysis in the field: To test hypothesis 2A, GoPro cameras with infrared capability will be deployed via SCUBA at sites where black sea bass are routinely observed to record behavior. Cameras will be positioned in a waterproof housing that is strapped to a cinder block. The cinder block will act as an anchor, and a small buoy will be attached to the block so that the camera can be relocated from the surface. Behavioral traits that will be quantified include shelter usage, site fidelity, feeding, and

4 aggressive behavior towards native species such as cunner and sea ravens. Two cameras will be deployed at each site for a period of 24 hours during survey sampling. Diet analysis: To test hypothesis 2B (that the range expansion of black sea bass is negatively impacting lobster populations), black sea bass samples will be collected from recreational and commercial fishermen in Maine and Massachusetts, as well as from the Massachusetts Division of Marine Fisheries trawl survey, to examine feeding habits within the GOM. The date, location, depth, and habitat type will be recorded at each site where fish are collected. Fish will be processed immediately, or frozen for later analysis. Processing will consist of measuring the total length and weight of each fish, then removing the stomach and collecting a muscle tissue sample. Stomachs will be dissected and contents will be counted and identified to species, if possible. Tissue samples will be dried in a drying oven, ground into a fine powder and weighed. These samples will then be analyzed for carbon and nitrogen stable isotope ratios (Sherwood and Grabowski 2010). Stable isotope nitrogen ratios can be used to determine the trophic position of an organism (Zanden and Rasmussen 1999) and stable isotope carbon signatures can be used to determine the source of carbon (e.g., benthic, demersal, pelagic, etc.) in marine food webs (Sherwood and Rose 2005). Stomach content data combined with stable isotope signatures will be used to compare the relative feeding habits of black sea bass within the GOM and southern New England. Figure 1. Three regions (Maine (midcoast ME), Massachusetts (MA) and Rhode Island (RI)) where SCUBA surveys will be conducted to quantify black sea bass abundance, distribution, and size frequency as well as observe behavior. Preliminary and Anticipated Results We conducted SCUBA surveys in Maine, Massachusetts, and Rhode Island between May and October in Our data suggests that black sea bass abundance decreases from south to north in correlation with decreasing water temperatures. This is not surprising given that sea bass are typically found in waters ranging from C (Steimle et al. 1999), which is a temperature range more commonly found in the southern GOM and south of Cape Cod. However, the proportion of juvenile to

5 adult sea bass differed greatly between regions. We found no evidence of juvenile sea bass at the survey sites in Maine, but a greater amount of adults than we found in Massachusetts. Meanwhile, Massachusetts was dominated by juveniles and Rhode Island had a mixture of both juveniles and adults. These observed differences may be due to juvenile and adult sea bass utilizing different habitats (Drohan et al. 2007) or responding to changes in water temperature differently. It is also possible that seasonal fluctuations in abundance differ between regions. Furthermore, distribution data collected via SCUBA may be misrepresentative of the actual population because dive surveys cannot encompass every type of habitat where sea bass may be found. However, the SCUBA surveys were designed to maximize both the habitats and temperature ranges sampled by surveying multiple transects within each site and multiple sites within each region over the course of several seasons. With the additional sampling efforts conducted over the next year, we anticipate being able to determine the patterns of sea bass distribution among the three regions, and assist efforts to forecast how those patterns will change as water temperatures continue to warm. We expect black sea bass will remain within a small home range area and exhibit territorial behavior. Sea bass have been shown to have high site fidelity and will aggressively defend their territory (Nelson et al. 2003, Drohan et al. 2007). Video analysis of sea bass in the wild may reveal how they influence native species and may aid in our diet study if we are able to capture predation events. Since very few native species in the GOM exhibit strong territorial behavior, we anticipate that black sea bass could potentially have large behaviorally-mediated effects on community structure. In regard to the diet study, we anticipate that black sea bass diet will consist largely of decapod crustaceans, similar to what was shown by Garrison and Link (2000). However, samples collected from the northern GOM, specifically the midcoast Maine region, will likely have a higher percentage of juvenile lobsters in their diet since this area has some of the highest population densities of juvenile lobsters in New England (Wahle and Incze 1997, Cowan et al. 2001, Steneck and Wilson 2001). Nelson et al. (2006) showed that lobsters comprised a higher percentage of striped bass diet in nearshore rocky habitats in Massachusetts where high densities of lobsters are known to occur. Our SCUBA surveys have confirmed that sea bass are utilizing similar nearshore rocky habitats in midcoast Maine. In addition, we have collected several sea bass samples from lobster fishermen in Maine who have caught them as bycatch in their lobster traps. This further suggests that black sea bass and lobster habitats overlap, and that they may be negatively impacting lobster populations in their newly expanded range. Significance of Research for Marine Conservation Currently, there is not a commercial or recreational fishery for sea bass north of Massachusetts and there are no regulations (i.e., quantity and size limits) on the species in coastal waters of Maine and New Hampshire. A better understanding of the distribution and abundance of sea bass in areas outside of their historically known range may aid in future stock assessment and management of the species. In addition, observing sea bass behavior will shed light on how they are impacting native species in the northern areas where sea bass have historically been less common. This may be particularly important for species with overlapping niches (e.g., cunner). Exposure to an aggressive, territorial species may result in a reduced ability to feed or find shelter, which could negatively impact native populations. If black sea bass abundance continues to increase in the northern GOM, negative impacts to the lobster population may also result. The lobster fishery is the second most valuable fishery in the United States (Fisheries of the United States 2011) and plays a crucial role in Maine s economy. An increase in predation of small juvenile lobsters could be detrimental to the fishery. Determining regional, seasonal, habitat-related, and size-related feeding habits of black sea bass in the GOM will enhance our ability to predict the impacts of the northern range expansion of black sea bass on benthic communities and key fisheries species. This project provides a framework for monitoring emergent species in the GOM, which will become increasingly important as sea water temperature continues to warm. Furthermore, understanding how emergent species influence local ecology will be crucial as regions such as the GOM shift to an ecosystem approach to fisheries management.

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