Dynamics of summer flounder, Paralichthys dentatus, seasonal ...

Estuarine, Coastal and Shelf Science 74 (2007) 119e130 www.elsevier.com/locate/ecss

Dynamics of summer flounder, Paralichthys dentatus, seasonal migrations based on ultrasonic telemetry Dana K. Sackett, Kenneth W. Able*, Thomas M. Grothues Marine Field Station, Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, 800 c/o 132 Great Bay Boulevard, Tuckerton, NJ 08087-2004, USA Received 26 April 2006; accepted 29 March 2007 Available online 18 June 2007

Abstract Migrations of summer flounder, Paralichthys dentatus, to and from estuaries to the continental shelf in the Mid-Atlantic Bight (MAB) occur seasonally but their dynamics are poorly understood. Ultrasonic telemetry, both passive and active, was used during 2003e2005 to determine timing and rate of juvenile and adult summer flounder (268e535 mm TL) migrating to and from the Mullica RivereGreat Bay estuary in southern New Jersey. Additionally, 7 years of inner continental shelf surveys off New Jersey were used to assess complementary seasonal movements. Most tagged fish emigrated from the estuary between July and September, though emigration lasted into December and appeared to be influenced by a number of factors. In July 2004, more tagged fish emigrated, at increased rates of movement, at low barometric pressure during a storm event. Trawl collections on the inner shelf demonstrated the same approximate immigration times as seen with telemetry. Later in the fall, increased numbers of tagged summer flounder emigrated from the estuary when dissolved oxygen was decreasing. Fall trawl surveys showed increased numbers of fish on the inner shelf when dissolved oxygen was decreasing in the Mullica RivereGreat Bay estuary, supporting the telemetry results. Fish emigrated from the estuary during the day and night but nighttime movements were in deeper water at slightly slower rates of movement. Exit and re-entry also occurred during the fall emigration. Ultrasonically tagged individuals demonstrated homing by returning to the same estuary, in March through June, in the second and third year of the study (39e6%, respectively). In summary, immigration may result from homing for a large proportion of summer flounder. Emigration may be associated with storms on an episodic scale, and dissolved oxygen and temperature on a seasonal scale. Ó 2007 Published by Elsevier Ltd. Keywords: estuary; summer flounder; ultrasonic telemetry; movement; migration; USA; New Jersey; Mullica RivereGreat Bay

1. Introduction Summer flounder, Paralichthys dentatus, inhabit continental shelf and estuarine waters from Nova Scotia to Florida (Wilk et al., 1980; Klein-MacPhee, 2002), although the majority of the population occurs in the Mid-Atlantic Bight (MAB, Massachusetts to North Carolina) (Wilk et al., 1980; Grosslein and Azarovitz, 1982). Juveniles and adults are reported to move into coastal and estuarine waters in the spring, remain throughout the summer, and emigrate in fall to the outer-continental shelf to overwinter (Westman and Neville, 1946; * Corresponding author. E-mail address: [email protected] (K.W. Able). 0272-7714/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.ecss.2007.03.027

Smith and Daiber, 1977; Wilk et al., 1980). However, the precise timing of migration, especially to and from estuaries, and how it varies with environmental factors are still vague or unknown (Packer et al., 1999). Spawning occurs during the fall emigration as fish move over the continental shelf (Able et al., 1990). The need for a better understanding of the migration dynamics of this species is necessary for fisheries management in relation to Essential Fish Habitat (EFH) (NOAA, 1996) and stock assessment for this highly valuable species (Kraus and Musick, 2001). Previous studies in the Mullica RivereGreat Bay estuary, a portion of the Jacques Cousteau National Estuarine Research Reserve (JCNERR) in southern New Jersey, make this region favorable for further examination of summer flounder migration

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dynamics because: (1) summer flounder are seasonally abundant and many aspects of their life history are relatively well known (Szedlmayer and Able, 1993; Able and Kaiser, 1994; Able, 1999), (2) the JCNERR provides continuous measures of environmental variables with dataloggers (Kennish and O’Donnell, 2002), and (3) the infrastructure for passive ultrasonic telemetry is in place (Grothues et al., 2005). The objectives of this study were: (1) to provide a detailed examination of summer flounder migration to and from the estuary on a monthly, tidal, and diel basis based on ultrasonically tagged fish, (2) to evaluate potential environmental influences on these migrations, and (3) to compare estuarine immigration and emigration with their seasonal distribution and abundance on the adjacent continental shelf off New Jersey.

2. Methods 2.1. Study site The Mullica RivereGreat Bay estuary in southern New Jersey is part of the JCNERR (Fig. 1). This system includes more then 27,000 ha of open water consisting of Great Bay, Little Egg Harbor, several back bays, the Mullica River, and its two smaller tributaries (Bass River and Wading River). The Pinelands National Reserve surrounds and protects the upper portion of the Mullica RivereGreat Bay watershed from anthropogenic sources of pollution and contributes to the

relatively unaltered nature of the reserve (Good and Good, 1984; Kennish and O’Donnell, 2002). Downstream, the estuary is bordered by extensive salt marsh (Spartina alterniflora). This estuary is typical of other MAB estuaries in that it experiences a wide annual temperature range (2 to 28 C), and moderate tidal range ( 0.05), and negatively correlated with dissolved oxygen (r ¼ 0.97, P > 0.05) and barometric pressure (r ¼ 0.82, P > 0.05). For the latter two, the relationship of both variables with the number of fish emigrating was opposite but strong (Table 3). The inverse relationship between the number of fish that emigrated from the estuary and low barometric pressure, as associated with a storm over several days (Table 3), could potentially explain the annual difference in emigration time (Fig. 7). In July 2004, 7 of the 10 fish that emigrated did so during an 11-day period when barometric pressure dropped below 1014 mbars (Fig. 7). Additionally, following this episode another six fish left as barometric pressure dropped again. Only one of these fish returned to the study site for a single day in December. A large increase in MDPH was associated with decreased barometric pressure demonstrating fish emigrated at faster rates when barometric pressure was lower (Fig. 6). The hourly rates of movement during emigration (MDPH) were significantly correlated with the barometric pressure recorded during those movements (r ¼ 0.39, P < 0.05) (Table 3). 3.4. Seasonal distribution and abundance on the adjacent continental shelf Summer flounder were also present on the continental shelf over the same seasons that they occurred in the estuary. Patterns of summer flounder abundance on the adjacent continental shelf varied with the same seasonal patterns as in the study estuary (Fig. 8). Abundance was lowest in winter, higher in spring and highest in summer and fall. Seasonal changes in abundance (CPUE) on the New Jersey inner continental shelf ( 0.05] in spring and fall. Summer (JuneeAugust) surveys had the highest abundance in shallowest and intermediate depths. Fall (SeptembereOctober) had high abundance across all depths. During the spring, summer, and fall when the summer flounder distribution was centered inshore, salinity was lower and temperature was higher inshore than offshore. Abundance correlated significantly and positively with temperature and


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Fig. 6. Number of telemetered summer flounder that emigrated from the estuary, in 2003 and 2004 combined, in relation to temperature (a) and barometric pressure (b), and the mean rate of tagged fish movement, measured by minimum displacement per hour (MDPH), in relation to barometric pressure in 2003 and 2004 (c). Error bars ¼ 1 standard deviation.

significantly inversely with salinity (Table 4). The few summer flounder collected in winter were within mean temperatures of 5.5 C (1.26 C) and salinities of 32.7 (0.82). In spring, the overall abundance increased from winter and the Table 3 Pearson’s correlation coefficients of environmental variables in relation to the number of fish emigrating from the estuary in each month and rate of emigration as measured by minimum displacement per hour (MDPH) averaged by individual in 2003 and 2004. Probability (P) is in parentheses; bold values indicate significance (Prob jRj under H0:r ¼ 0) Variables

Number of emigrating fish per month (JulyeDecember)

MDPH (m/h) Temperature ( C) Salinity Dissolved oxygen (mg/L) Depth (m) Photoperiod (h) Barometric pressure (mbars)

0.57 0.85 0.94 0.97

Rate of emigration (MDPH in m/h)

(0.24) (0.03) (0.02) (0.01)

0.21 (0.22) 0.06 (0.73) 0.07 (0.71)

0.95 (0.20) 0.87 (0.03) 0.82 (0.05)

0.44 (0.33) 0.40 (0.02) 0.39 (0.02)

highest abundance was inshore within mean temperatures of 8.0 C (1.6) and salinities of 30.9 (1.4). In summer, highest abundance was inshore in temperatures of 18.7 C (3.5) and salinities of 30.7 (1.1). In fall, highest inshore abundance was at temperatures of 16.7 C (2.24) and salinities of 31.0 (0.9) while estuarine temperatures were approximately 16.1 C (3.6). In fall, the abundance on the inner continental shelf was high when dissolved oxygen was higher. At the same time dissolved oxygen in the Mullica Rivere Great Bay estuary was lower (r ¼ 0.23, P < 0.05) (Table 4). Thus, fish moved out of New Jersey estuaries and onto the shelf when estuarine dissolved oxygen was lower than in the ocean. 4. Discussion 4.1. Patterns of migration Evidence of homing for summer flounder was demonstrated in this study with return percentages of 39% and 6% in the second and third year of study, respectively. The 6% return


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Fig. 7. Percent of fish emigrating relative to the change in mean barometric pressure during July 2003 (a) and 2004 (b).

rate recorded in 2004 underestimates true returns because battery life was conservatively expected to expire in winter of 2003/2004 (Table 1), although some lasted longer. Tagged fish that returned in 2004 with an expired transmitter would not have been detected. The occurrence of summer flounder returning to a particular estuary in subsequent years (homing) was previously documented by markerecapture studies in New York (Poole, 1962), New Jersey (Hamer and Lux, 1962; Murawski, 1970), and Maryland (Jesien et al., 1992). These values range from 3% in the HudsoneRaritan estuary (Hamer and Lux, 1962) to 25e53% in various New Yorke New Jersey estuaries after 2e3 years (Poole, 1962). The high percentages of summer flounder homing reported in this and the other studies noted have important implications for fisheries management and stock assessment. For example, if summer flounder consistently return to the same estuary, stock depletion or rebuilding is under local influence regardless of whether factors are positive (habitat restoration) or negative (e.g. overfishing and habitat destruction). In addition, locally replenished or depleted estuarine populations could lead to stronger than expected effects on continental shelf metapopulations (Ray, 2005). The broad seasonal inshore/offshore patterns of summer flounder movement/migration observed in this study from the inner shelf otter trawl survey and from fish tagged in the estuary were consistent with the general patterns assumed by previous investigations (Westman and Neville, 1946; Poole, 1962; Able and Kaiser, 1994; Packer and Hoff, 1999). As a result of these studies, it is clear that the majority of the MAB

summer flounder population is offshore, on the outer-continental shelf, in winter. Summer flounder appear to have immigrated into New Jersey inner continental shelf waters in March since fish were not present in the February otter trawl surveys, some were detected in March, and others were collected close inshore during April. Continued shoreward migration in the spring is consistent with the increased summer flounder abundance in continental shelf waters in June and August. The timing of tagged summer flounder immigration into the estuary in the spring agreed with the patterns observed from seasonal otter trawl collection on the inner continental shelf. Three of the seven fish tagged in 2003 were recorded reentering the estuary in 2004 from MarcheJune. Tagging studies in Chesapeake Bay found summer flounder immigration began as early as February, but immigration is generally thought to begin in March and continue until June (Desfosse, 1995). Thus, there is general similarity in the timing of immigration in this study with other estuaries in the MAB. The fall decrease in summer flounder abundance on the inner continental shelf in September and October coincided with a wide distribution across the continental shelf (Packer and Hoff, 1999) and thus demonstrated emigration from coastal waters that resulted in few or no fish in New Jersey inner continental shelf waters in winter. Other studies from this site found emigration from estuarine habitats in Auguste December (Able et al., 1990; Rountree and Able, 1992; Szedlmayer et al., 1992; Szedlmayer and Able, 1993). However, in this study emigration from the estuary began at different times in different years. Emigration of tagged individuals began in

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August in 2003 and in July for 2004. The main difference in the emigration time between years seemed to be the much higher percentage of fish that emigrated in July 2004. Estuarine emigration timing appears to vary with latitude. In the north, summer flounder were collected in an estuary into October (Newark Bay, New Jersey, Wilk et al., 1977) and December (HudsoneRaritan estuaries, Reid et al., 1999). South of the study site, summer flounder were captured year round in Delaware Bay (Smith and Daiber, 1977) but they were abundant only into November. In Chesapeake Bay summer flounder emigration began in October and proceeded until December (Desfosse, 1995). Thus, temporal and spatial variability in the exact timing of summer flounder estuarine emigration exists in the MAB and demonstrates the strong probability that dynamic factors influence this timing. 4.2. Factors influencing migration

Fig. 8. Mean abundance (CPUE) of summer flounder across depth strata on the inner continental shelf off New Jersey relative to bottom temperatures during the spring (a), summer (b), fall (c), and winter (d). Error bars ¼ 1 standard deviation. See Fig. 4 for locations of trawl samples and depth categories.

Evidence that dynamic environmental variables influenced annual emigration was seen in this study because, although emigration timing was different between years, the environmental variables recorded during each fish’s emigration were not. The episodic variable barometric pressure and the seasonal variables dissolved oxygen and temperature appeared to correlate best with estuarine emigration in this study. For example, changing barometric pressure, associated with storms, was strongly correlated with emigration from the estuary and could explain the difference in emigration patterns between 2003 and 2004. Additionally, the mean rate of movement during storm-related emigration was dramatically faster in July (4448.7 m/h) than in August (1289.2 m/h) or September (171.6 m/h). Rate of movement is a direct biologically meaningful measure of migratory behavior because higher rates of movement would have higher energetic costs (Zabel, 2002). Temperature, and presumably its effect on metabolic rate, was not significantly correlated with rate of movement during emigration over this time. However, barometric pressure, recorded only during the hours that tagged fish were emigrating, was significantly and strongly negatively correlated to the number of fish that left the estuary and their

Table 4 Pearson’s correlation coefficients of environmental variables recorded at Little Egg Inlet and at the site of otter trawl tows in relation to seasonal summer flounder abundance (CPUE) from 1996 to 2003. System Wide Monitoring Program (SWMP) variables were recorded near Little Egg Inlet with SWMP datalogger 126 at the same date and time as the otter trawl surveys. Bold values indicate Prob > jRj under H0:r ¼ 0; significance, P-value < 0.05; N, number of otter trawl tow. See Fig. 4 for locations of trawl tows and SWMP datalogger 126 Variables

Spring abundance CPUE (n ¼ 315)

Summer abundance CPUE (n ¼ 587)

Fall abundance CPUE (n ¼ 320)

Winter abundance CPUE (n ¼ 243)

Ocean depth (m) Ocean otter trawl temperature ( C) Ocean otter trawl salinity Estuarine SWMP temperature ( C) Estuarine SWMP salinity Estuarine SWMP dissolved oxygen (mg/L) Estuarine barometric pressure (mbars)

0.39 (