Spatial and Temporal Variation in Recent Growth ...

Estuaries

Vol. 23, No. 1, p. 10–20

February 2000

Spatial and Temporal Variation in Recent Growth, Overall Growth, and Mortality of Juvenile Weakfish (Cynoscion regalis) in Delaware Bay R. PAPERNO1 T. E. TARGETT P. A. GRECAY 2 University of Delaware Graduate College of Marine Studies Lewes, Delaware 19958 ABSTRACT: We examined relative abundance of juvenile weakfish, Cynoscion regalis, collected during 1986 and 1987 and tested for spatial differences in growth and survival within Delaware Bay. Juvenile weakfish recruit to all areas of Delaware Bay, and two cohorts were present during each year of the study. Although catch per unit effort (CPUE) varied among areas within the bay, there was a general trend of higher CPUE at lower salinities; abundance quickly declined near the end of September in all areas of the bay. Estimated growth rates from otolith increment analysis of juvenile weakfish ranged from 0.69 mm d21 to 0.97 mm d21. Spatial and temporal patterns in recent growth rate followed a general pattern: highest in the middle bay, lowest in the upper bay, and intermediate in the lower bay. Mortality rates were usually lowest in the low salinity region of the middle and upper bay during both years. There was no difference in mortality between cohorts in the middle bay, while in the upper bay the later-spawned fish had lower mortality and in the lower bay the early-spawned fish had lower mortality. Analysis of spatial and temporal patterns in growth and mortality suggests that there is a seasonal trade-off between habitat usage and resource availability for juvenile weakfish. The function of oligohaline and mesohaline waters as optimal nursery areas (in terms of growth and survival) changes due to the seasonally dynamic physicochemical characteristics in Delaware Bay.

sonally (Pennock and Sharp 1986; Fogel et al. 1992; Garvine et al. 1992). Salinity in Delaware Bay can range from 0‰ to 30‰ and temperature can vary by as much as 4–68C from the upper to lower bay (Garvine et al. 1992; Grecay and Targett 1996a). Laboratory experiments have demonstrated that both temperature and salinity affect feeding and growth rates in juvenile weakfish (Lankford and Targett 1994). This research suggested maximum growth potential occurred at mesoholine salinities. Field work has provided evidence of lower condition and feeding in juvenile weakfish from the oligohaline areas of Delaware Bay. This work suggested that the value (in terms of growth and survival) of the nursery grounds varied spatially (Grecay and Targett 1996a). The objectives of this study were to determine the spatial patterns of relative abundance, recent growth, overall growth, and mortality of juvenile weakfish in Delaware Bay. These patterns were compared with synoptic observations of patterns in condition and feeding for this species and in light of the laboratory evidence regarding optimal nursery conditions. We also speculate on environmental conditions that may explain observed patterns in growth and survival.

Introduction The east coast of the United States is characterized by many estuarine systems that serve as critical nursery habitats for juvenile fishes (Chao and Musick 1977; Weinstein 1979; Able and Fahay 1998; Able et al. 1998). The functional importance of these estuarine waters for juvenile fishes has been attributed to the abundance of prey resources, reduction of predators, and more recently, physicochemical suitability for growth (Miller et al. 1985; Lankford and Targett 1994; Szedlmayer and Able 1996; Wantiez et al. 1996). In the mid-Atlantic region, Delaware Bay serves as an important nursery ground for juvenile weakfish, Cynoscion regalis (Thomas 1971; Grimes 1984; Lankford and Targett 1994; Grecay and Targett 1996a). The heterogeneity of estuaries such as Delaware Bay in terms of biological and physicochemical gradients can be pronounced spatially and sea1 Corresponding author: Present address: Florida Fish and Wildlife Conservation Commission, Florida Marine Research Institute, 1220 Prospect St., STE 285, Melbourne, Florida 32901; tele: 321/984-4828; fax: 321/984-4824; e-mail: [email protected]. 2 Present address: Department of Biological Sciences, Salisbury State University, Salisbury, Maryland 21801.

Q 2000 Estuarine Research Federation

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Growth and Mortality of Juvenile Weakfish

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Fig. 2. Relationship between standard length (SL) and otolith radius (M) of juvenile weakfish (Cynoscion regalis).

Fig. 1. Sampling stations in the Delaware Bay estuary. Stations 1 and 2 are the lower bay; stations 3 and 4 are the middle bay; stations 5 through 7 are the upper bay.

Materials and Methods Juvenile weakfish were collected with an otter trawl (width 5 4.6 m; 2.5-cm stretch mesh body, 19-mm stretch mesh cod end, 3.2-mm mesh cod end liner) from early July to November in 1986 and from early July to early October in 1987. Collections were made at seven stations distributed over 85 km, from the mouth of the bay to the Chesapeake-Delaware Canal (Fig. 1). Trawls were towed for 10 min at each station and repeated tows were taken until at least 40 juveniles were collected. Fish were weighed to the nearest 0.1 g and measured to the nearest 0.5 mm standard length (SL). A subsample was taken from the catch at each station by dividing the catch into 5-mm size classes (40–44, 45–49, 50–54, 55–59, 60–64, 65–69, and 70–74) and up to 10 fish were taken (evenly distributed) within each size class. The bay was divided into three areas by grouping stations one and two (lower bay), stations three and four (middle bay), and stations five, six, and seven (upper bay) for all analyses of spatial patterns. From these respective bay areas, 108, 113, and 163 fish in 1986 and 124, 146,

and 204 fish in 1987 were subsampled for growth and mortality analyses. In all cases the length-frequency values were standardized to represent equivalent catch per unit effort (CPUE; number captured in three 10-min tows at 1,600 m tow21). Tow distance was calculated using LORAN C. Otoliths were removed and prepared using the methods described by Neilson and Geen (1981). Saccular otoliths (sagitta) were embedded in thermoplastic glue on a glass slide, ground on one side with 600 grit wet-dry carborundum paper, and polished with 0.3-mm alumina oxide. Each otolith was then removed, attached to a slide with the polished face down, and the exposed surface was polished to the nucleus. All increment measurements were taken on a transverse section through the nucleus of the otolith at 4503 with the aid of an Olympus Cue 2 Image Analysis System. Growth rates were examined with several methods to evaluate short-term and seasonal trends in cohort growth rates. Growth rates reported in this study represent the growth of the average individual (cohort) rather than a single individual. Recent growth rate (mm d21) for the week prior to capture was calculated for each fish by measuring the width of the outer seven increments and employing the following equation: G 5 ((R2L/M) 2 (R1L/M))/7 where G is the growth rate during the last 7 d, R1 is the otolith radius to the seventh-to-last increment, R2 is the otolith radius to the last full increment, L is the fish length in mm, M is the maxi-

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Fig. 3. Standardized length-frequency distribution of age-0 weakfish (Cynoscion regalis) in Delaware Bay 1986, by area and sampling date (CPUE 5 number captured in three 10-min tows at 1,600 m tow21).

mum otolith radius along the line of measurements, and 7 is the period in days. Otolith radii were measured to the nearest 1.0 mm. Mean recent growth rate data were analyzed with general linear models and PROC GLM with a Tukey’s multiple comparison test (a 5 0.05) when appropriate (SAS Institute 1988). Overall growth rate (mm d21) was calculated for each cohort separately with a linear model to describe the relationship between age and standard length of juveniles in each subsample. Analysis of covariance was used to compare the slopes of the

regression lines and Tukey’s multiple comparison test was used to determine where differences between slopes existed (Zar 1984). Growth rates were also compared among areas of the bay by back-calculating to length at age 30 d and then comparing lengths with ANOVA and Tukey’s multiple comparison tests (a 5 0.05). Because the maximum radius of the sagitta has a linear relationship with weakfish standard length (Fig. 2), back-calculated standard length at age 30 d was determined by the equation: SL30 5 (R30 3 Slc)/M

Growth and Mortality of Juvenile Weakfish

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Fig. 4. Standardized length-frequency distribution of age-0 weakfish (Cynoscion regalis) in Delaware Bay 1987, by area and sampling date (CPUE 5 number captured in three 10-min tows at 1,600 m tow21).

where SL30 is the back-calculated standard length at age 30, R30 is the otolith radius at 30 d, Slc is the standard length at capture, and M is the otolith radius at capture. Each radius value represents the mean of three measurements. Relative survival may be the best way to avoid the problems of emigration and differential gear selection with age. Relative survival was estimated using Eq. 3 in Hoenig et al. (1990): Loge(Rt) 5 loge(q) 1 (ZE 2 ZL)t where q is the intercept and comprises the catch-

ability coefficients and initial abundances, ZE and ZL are the instantaneous mortality rates of fish from area/group one and area/group two, respectively. The resultant slope (ZE 2 ZL) of the regression of the ratio of abundance of two areas/groups (Rt) is equal to an estimate of the difference in the instantaneous mortality rates. Mortality rates were compared within cohorts from different bay areas, and then using the 1987 data, compared between cohorts (groups) among areas. Under the assumptions that the ratio of catchabilities is constant over time and that there is a constant difference in in-

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TABLE 1. Recent growth rate (least-squares mean values in mm d21; 6 SE) during the 7 days prior to capture of juvenile weakfish (Cynoscion regalis) among location in Delaware bay in 1986 and 1987. Numbers in parentheses are sample sizes. Values with the same superscript are not significant different at p , 0.05, * p , 0.05, ** p , 0.01, *** p , 0.001, NA 5 size classes in which no fish were collected. Bay Area Size Class

Lower

Middle

40–44 45–49 50–54 55–59 60–64

0.60 6 0.05 (7)a 0.68 6 0.03 (8)a 0.91 6 0.06 (9)a NA 1.03 6 0.10 (5)

Early July 1986 0.86 6 0.05 0.92 6 0.04 0.98 6 0.06 0.96 6 0.03 0.96 6 0.09

55–59 60–64 65–69 75–79

0.70 6 0.06 (4)a 0.79 6 0.05 (17)a 0.79 6 0.04 (13)a NA

85–89

Upper

0.53 0.56 0.69 0.79 0.76

6 6 6 6 6

0.02 0.02 0.05 0.06 0.09

(43)a*** (26)c*** (14)b** (2)* (7)ns

Late July 1986 1.09 (1)b 1.05 6 0.09 (4)b 1.02 6 0.07 (4)b 0.99 6 0.08 (15)

0.63 0.70 0.68 0.71

6 6 6 6

0.03 0.06 0.05 0.06

(13)a*** (11)a* (8)a** (6)***

0.88 6 0.05 (13)

Mid-July 1987 0.84 6 0.09 (11)

0.82 6 0.04 (10)ns

NA

Late July 1987 0.78 6 0.03 (17)

0.67 6 0.04 (11)*

45–49 50–54

0.72 6 0.04 (4) 0.76 6 0.04 (4)a

Late August 1987 0.68 6 0.03 (10) 0.70 6 0.03 (10)a

75–79

1.00 6 0.03 (11)

Early September 1987 NA

0.83 6 0.03 (12)**

80–84

0.77 6 0.07 (3)

Late September 1987 0.82 6 0.04 (10)

0.73 6 0.03 (24)ns

85–89

stantaneous mortality rates (parallel slopes of the catch curves), the plot of the ratio of catches versus time would be expected to be linear. Results Examination of length frequency from 1986 and 1987 revealed the presence of two cohorts of juvenile weakfish each year. Abundance in 1986 was highest in July in all areas of the bay and fish were abundant throughout the middle and upper estuary through early August (Fig. 3). The distribution and abundance during 1987 was different from the pattern observed in 1986. While juvenile weakfish were collected from all areas of the bay, they were only abundant at the upper bay stations and abundance remained high in the upper bay through September (Fig. 4). Recent growth rate in 1986 was significantly (p , 0.05) slower in the upper bay area and faster in the middle bay area in all but the 60–64 mm size class caught in early July (Table 1). The growth rate in the lower bay was more variable than in the other areas examined. The pattern of recent growth was less defined for the size classes examined from 1987. For all 1987 comparisons the pattern was one of slowest growth in the upper bay, but not always significantly so. In 1987, recent

(8)b (10)b (11)a (10) (6)

0.61 6 0.02 (32)ns 0.64 6 0.01 (37)b**

growth rates in the lower bay were more often greater than those in the middle bay area (Table 1). Significant differences were found in overall growth rates of the first cohort of 1986 among the sampling areas (p , 0.05, ANCOVA). Greatest overall growth occurred in the middle and upper bay (p , 0.05, b 5 0.97 mm d21 and b 5 0.93 mm d21, respectively; Table 2 and Fig. 5). Back-calculated length at age 30 d was greater in the lower and middle bay in 1986 (Table 3). The second cohort of fish was not represented well enough in the 1986 samples to make reliable estimates of growth or mortality. Significant differences were found in overall growth rates in the first cohort in 1987 (p , 0.001, ANCOVA). Overall growth rate varied among areas with the highest growth occurring in the upper bay (p , 0.01, Tukey’s, b 5 0.90 mm d21; Table 2 and Fig. 5). Back-calculated length-at-age 30 d for these fish was greatest in the lower bay (Table 4). Overall growth rates within the second cohort from 1987 also varied among areas (p , 0.001, ANCOVA). Again, as with the first 1987 cohort, the fastest growth rate occurred in the upper bay (p , 0.01, Tukey’s, b 5 0.92 mm d21; Table 2 and Fig. 5). Back-calculated length at age 30 d for these fish

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Growth and Mortality of Juvenile Weakfish

TABLE 2. Results of ANCOVA and Tukey’s multiple range test of overall growth rates (mm d21) (6SE) of weakfish among sampling areas in Delaware Bay. Correlation coefficients are included in parentheses. Comparisons with the same letter are not significantly different at p , 0.05, * p , 0.05, ** p , 0.001. Growth Rate (6SE) (r2) 1986 Cohort 1

Upper Middle Lower F-value

0.93 6 0.02 (0.988) 0.97 6 0.01 (0.989) 0.88 6 0.01 (0.986) 11.33*

1987 Cohort 1 ab b a

was greatest in the middle and lower bay (Table 5). In general, the estimated mortality rates for juvenile weakfish in Delaware Bay were lowest in the lower salinity region of the middle and upper bay during both years. The coefficients of determination for six of nine area comparisons were good (r2 5 0.63 to 0.99), while only three comparisons were poor (r2 5 0.31 to 0.38; Fig. 6). This suggests that we can put some confidence in the estimates of differential mortality. In 1987, comparisons between cohorts were different in each area of the bay (Fig. 7). In the lower bay, the first cohort experienced lower mortality than the second cohort. In the middle bay, there was no difference in mortality between cohorts, and in the upper bay the

0.90 6 0.02 (0.889) 0.69 6 0.01 (0.960) 0.80 6 0.01 (0.967) 47.31**

1987 Cohort 2 a b c

0.92 6 0.01 (0.988) 0.74 6 0.02 (0.923) 0.74 6 0.01 (0.986) 38.56**

a b b

first cohort experienced higher mortality than the second cohort. The coefficient of determination for only the upper bay was sufficient (r2 5 0.78) to make confident comparisons. Discussion The seasonal pattern of juvenile weakfish occurrence in Delaware Bay is consistent with previous reports of occurrence in the Mid Atlantic Bight (see Mercer 1983 for review; Szedlmayer et al. 1990). An early spawned cohort of juvenile weakfish was present in the beginning of July, with a second cohort appearing in late July to early August. This pattern was similar in 1986 and 1987. Juveniles recruit to all areas of Delaware Bay, with the heaviest recruitment occurring in areas of

Fig. 5. Overall growth in standard length of each cohort of age-0 weakfish (Cynoscion regalis) in Delaware Bay, July–November 1986 and July–October 1987, by area.

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TABLE 3. Comparison of back-calculated length at age 30 d for cohort 1 juvenile weakfish (Cynoscion regalis) among areas in Delaware Bay (1986). Values with the same letter are not significantly different at p , 0.05, ** p , 0.001. Source of Variation

Analysis of Variance Sum of Square df Mean Square

Between areas Within areas Total

410.33062 905.18573 1315.5164

2 115 117

205.16 7.87

F-ratio

26.065**

TABLE 4. Comparison of back-calculated length at age 30 d for cohort 1 juvenile weakfish (Cynoscion regalis) among areas in Delaware Bay (1987). Values with the same letter are not significantly different at p , 0.05, ** p , 0.001. Source of Variation

Analysis of Variance Sum of Squares df Mean Square

Between areas Within areas Total

916.50259 394.67624 1311.1788

Tukey’s multiple comparison test

2 121 123

458.25 3.26

F-ratio

140.491**

Tukey’s multiple comparison test

Area

Count

Average

Homogeneous group

Area

Count

Average

Homogeneous group

Upper Middle Lower

33 44 45

14.50 18.29 18.95

a b b

Upper Middle Lower

34 45 45

12.66 16.39 19.54

a b c

,20‰ salinity. Movement out of the area of initial recruitment is minimal, but fish move out of the estuary in appreciable numbers by October (Grecay and Targett 1996a). Based on analysis of length-frequency data, it has been suggested that juvenile weakfish recruit to the low salinity reaches of the York River system in Chesapeake Bay, Virginia, and migrate down-river as they grow (Chao and Musick 1977). In the same system, however, Szedlmayer et al. (1990) reported based on cohort distributions that juvenile weakfish moved up-river as they grow. Therefore, the movement pattern of juvenile weakfish in the York River nursery remains unclear. In Delaware Bay, spatial patterns in size stratification of juveniles evident in 1986 appear to be more the result of slower growth in the upper estuary than a ‘conveyor belt’ movement of juveniles down-estuary as they grow. Temporally stable patterns (higher growth in the low salinity areas) have been shown to exist in recent growth, back-calculated length-at-age, and condition factor among these areas for juvenile weakfish in Delaware Bay (Grecay and Targett 1996a; present study). In addition, the patterns in recent growth (measure of current conditions) were similar to longer term patterns of overall growth rates and condition. Congruency in patterns of recent growth, condition factor, and overall growth would likely not be established if fish moved extensively among bay areas. Furthermore, the movement of larger juveniles out of a particular area could obscure any changes in length frequency due to growth by the original recruits. The results presented here suggest the presence of new recruits from the second cohort in the upper and middle bay with no loss of larger individuals from the upper bay area. The production of multiple cohorts of juvenile weakfish may act as a mechanism to ensure survival in Delaware Bay as has been suggested for York River-Chesapeake Bay weakfish (Szedlmayer et al.

1990). Multiple cohorts of juveniles throughout the nurser y habitat may enhance year-class strength when environmental conditions exceed the range of tolerance and result in recruitment failure of at least one cohort (Lambert 1984; Lambert and Ware 1984; Szedlmayer et al. 1990). Back-calculation of fish size to examine changes in growth has been used extensively and it presumes some relationship between fish size and otolith size (or other hard part). When linked to species-specific evidence of the relationship between fish growth and otolith growth, this method is of greater utility. Several authors have demonstrated an uncoupling of otolith and somatic growth (Marshall and Parker 1982; Mosegaard et al. 1988; Wright et al. 1990). Although the relationship between ototlith and somatic growth may be complex, most studies support a close correspondence (Volk et al. 1984; Maillet and Checkley 1989; McMichael and Peters 1989; Chambers and Miler 1995; Fitzhugh and Rice 1995; Paperno et al. 1997). Significant differences in increment width in response to slight changes in feeding were detectable after several days in juvenile weakfish (Paperno et TABLE 5. Comparison of back-calculated length at age 30 d for cohort 2 juvenile weakfish (Cynoscion regalis) among areas in Delaware Bay (1987). Values with the same letter are not significantly different at p , 0.05, ** p , 0.001. Source of Variation

Between areas Within areas Total

Analysis of Variance Sum of Square df Mean Square

74.02 631.40 1311.18

2 128 123

37.01 4.93

F-ratio

7.503**

Tukey’s multiple comparison test Area

Count

Average

Homogeneous group

Upper Middle Lower

44 43 44

15.61 16.89 17.39

a b b

Growth and Mortality of Juvenile Weakfish

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Fig. 6. Logarithm of the ratio (R) of abundance of juvenile weakfish (Areai11/Areai) versus sampling period (measured as sampling trip) in 1986 and 1987 for each cohort. Slopes of the linear regression lines estimate ZE 2 ZL. (Note that the scales of the ordinate vary among plots).

al. 1997). Condition factor and recent growth rate of juvenile weakfish in Delaware Bay showed similar patterns (Grecay and Targett 1996a; this study). Juvenile weakfish caught at middle bay stations had consistently higher condition factor and recent growth rates than those caught at upper bay stations in 1986 (Grecay and Targett 1996a; this study). Middle bay stations also had higher or equal recent growth than lower bay stations in 1986. There was no clear pattern in recent growth in 1987. The preferred prey of juvenile weakfish is the mysid shrimp Neomysis americana (Grecay and Targett 1996a; Lankford and Targett 1997). Walker (1989) showed that, although there were no significant spatial differences in mysid density in Delaware Bay in 1987, within a given area mysids could exhibit temporal and extreme, small-scale, spatial variability in density. Analyses of feeding patterns of juvenile weakfish in Delaware Bay showed that in the upper bay, mysids comprised only 53% of the items found, but in the middle bay mysids made up 91% of the diet (Grecay and Targett 1996a). Therefore, despite the ubiquitous occurrence of mysid shrimp, there were spatial differences in the amount of mysids found in the diet. The patterns of mysid consumption corresponded to patterns of turbidity and light levels, with high

turbidity and low light in the upper bay associated with reduced amounts of mysids found in the diet, and lower turbidity and higher light level in the middle and lower bay areas associated with increased percentage of mysids in the diet. Laboratory experiments have demonstrated that turbidity is a secondary factor to light availability and the effects (reduced feeding) are manifested when turbidity levels are high enough to eliminate light penetration near the bottom (Grecay and Targett 1996b). These condition of high turbidity and no light are found in parts of the oligohaline areas (upper bay) of Delaware Bay and field work has confirmed the reduction in feeding by weakfish in these areas (Grecay and Targett 1996a). Lankford and Targett (1994) provide a compelling argument that the oligohaline portion of Delaware Bay serves as the optimal nursery area during the early part of the juvenile residency. It is during this time when low temperature conditions allow fish to trade-off the energetic costs of tolerating the stressful conditions with increased survival due to lower predation pressure. Then as fish grow and become less vulnerable to predation, and seasonal temperatures increase making the energetic costs of oligohaline areas too great, weakfish presumably move toward the mesohaline areas of the bay. The growth:mortality (G:Z) ratios estimated for juvenile

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Fig. 7. Logarithm of the ratio (R) of abundance of juvenile weakfish (cohorti11/cohorti) versus sampling period (measured as sampling trip) in 1987 for each area. Slopes of the linear regression lines estimate ZE 2 ZL. (Note that the scales of the ordinate vary among plots).

weakfish are consistent with this hypothesis (Lankford and Targett, 1994, Paperno unpublished data). The patterns of weakfish growth and survival also follow trends seen in chlorophyll a and primary productivity, which reflect the spring bloom’s occurrence in the middle bay area (Pennock and Sharp 1986; Fogel et al. 1992). This similarity in spatial pattern within Delaware Bay suggests that growth and survival of juvenile weakfish may be more closely linked through the food chain than previously thought. Aging juvenile weakfish from otolith increments was previously believed to be unreliable (Szedlmayer et al. 1990). Validation of daily increment periodicity has allowed otolith increment techniques to be applied to this species (Paperno et al. 1997). Estimated growth rates of juvenile weakfish in Delaware Bay (0.69–0.97 mm d21) calculated by applying otolith increment analysis are similar to those (0.76–1.13 mm d21) calculated by scale increment analysis by Szedlmayer et al. (1990) for weakfish in the York River-Chesapeake Bay, Virginia. Similar growth rates have been calculated from otolith increment analysis for Cynoscion nebulosus from Tam-

pa Bay, Florida (0.46–0.76 mm d21; McMichael and Peters 1989) and from Galveston Bay, Texas (0.6 mm d21; Maceina et al. 1987). Growth rates calculated from length-frequency analysis of C. arenarius (1 mm d21; Shlossman and Chittenden 1981) and C. nothus (0.8–1.3 mm d21; Devries and Chittenden 1982) are also similar. Mortality rates of juvenile fishes have rarely been estimated (Weinstein and Walters 1981; Crecco et al. 1983; Weinstein 1983; Miller et al. 1985; Mansfield and Jude 1986; Hume and Parkinson 1988). While several studies have been done on sciaenid species (Weinstein and Walters 1981; Weinstein 1983; Currin et al. 1984; Miller et al. 1985), mortality rates have yet to be reported for any species in the genus Cynoscion. Daily mortality rates calculated for juvenile weakfish in Delaware Bay (2.2– 9.9% d21; unpublished data) are similar to mortality rates of juvenile spot (L. xanthurus; 2.0–6.1% d21) and Atlantic croaker (Micropogonias undulatus; 2.3% d21) in Pamlico Estuary, North Carolina (Weinstein and Walters 1981; Currin et al. 1984; Miller et al. 1985) and spot (2.3% d21) in the York River, Virginia (Weinstein 1983). In addition to having similar mortality rates as spot and Atlantic croaker, the general pattern of lower mortality in the low salinity areas that was described by Miller et al. (1985) for Pamlico Estuary, North Carolina, was also seen for juvenile weakfish in Delaware Bay. Many of the factors affecting mortality in young fish are similar to those controlling growth (Houde 1987). Low food availability results in slower growth, which in turn causes fish to be smaller and highly vulnerable to predation (Miller et al. 1985; Houde 1987). High growth rates can influence positively the survival of fish during the early life history stages. Post and Prankevicius (1987) demonstrated that young-of-the-year yellow perch that were faster growing and larger survived better than slower growing individuals. The pattern of mortality and growth in 1986 suggests that slower growth results in higher overall mortality for juvenile weakfish. In 1986, higher relative mortality was found in an area where fish had the slowest recent and overall growth, but as the fish became larger, mortality was lower in the areas of lower growth. In 1987, the pattern of higher growth and lower mortality occurred in the upper bay, suggesting that the physicochemical conditions were more conducive for growth and survival (Lankford and Targett 1994; this study). Early in the season during both years, mortality was greater in the lower bay than the mid bay. The lower bay is most similar to the adjacent ocean in its physical characteristics. In terms of predation, the lower bay would contain estuarine and oceanic predators, whereas in the upper bay with salinity

Growth and Mortality of Juvenile Weakfish

as low as 4‰, oceanic predators would be effectively excluded from the predator community (Rozas and Hackney 1984; Miller et al. 1985). Highly turbid waters, such as in the upper bay, that reduce visibility would further reduce the predation rate by visual feeders (Vinyard and O’Brien 1976; Blaber and Blaber 1980). Lower recent growth rates, lower length-at-age, and the general pattern of lower mortality in the upper bay suggest there may be a trade-off between optimizing growth and minimizing mortality. The oligohaline areas of estuaries such as Delaware Bay may serve as important seasonal components of the nursery habitat for weakfish (Lankford and Targett 1994). Taylor (1987) in an analysis of predator stomach contents concluded that predation pressure on juvenile weakfish was greatest in the middle and lower areas of Delaware Bay. While predation pressure in the middle and lower bay may result in the production of a few, fast-growing individuals, it may be beneficial for juvenile weakfish to have reduced growth rates in the upper bay. These juveniles would experience reduced predation, resulting in slightly smaller but more numerous individuals recruiting to the overwintering population. A review of the data collected by Weinstein and Walters (1981) shows a similar pattern of decreasing growth and mortality upriver for spot in the Cape Fear River estuary, North Carolina. From observed growth, mortality, and feeding rates of juvenile weakfish in Delaware Bay, it can be suggested that optimal conditions for growth and survival change spatially due to pronounced seasonal changes in biotic and physicochemical characteristics. ACKNOWLEDGMENTS We thank P. Rowe, D. Goshorn, A. Boettcher, W. Walker, A. Eklund, and K. Long who contributed time in the field and helped to complete this project successfully. We would also like to thank Dr. R. F. Shaw and two anonymous reviewers whose comments greatly improved this manuscript. This study was supported by a grant to T. E. Targett from the United States Fish and Wildlife Service, Federal Aid in Fisheries Restoration, Wallop-Breaux Fund (project F-36-R:1-3), through the Delaware Department of Natural Resources and Environmental Control.

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