1. Preliminary Assessment of the Age, Growth, & Reproduction of Sheepshead, Archosargus probatocephalus in the Mississippi Sound and Adjacent Waters
J. Read Hendon, William D. Dempster, Jason D. Tilley, Gary G. Gray, Nancy J. Brown-Peterson and James R. Warren
Center for Fisheries Research & Development , Gulf Coast Research Laboratory Department of Marine Sciences, Ocean Springs, Mississippi and Mississippi Department of Marine Resources, Biloxi, Mississippi
Abstract
Sheepshead, Archosargus probatocephalus, occurs in coastal waters of the western Atlantic Ocean from Nova Scotia to Brazil, with the densest populations found in the Gulf of Mexico. Sheepshead support an important winter recreational and commercial fishery in Mississippi. In light of this, we currently are collecting life history data on this species to gain a better understanding of its biology for use in any future management decisions. During , 241 sheepshead (102 males, size range mm TL; 122 females, size range mm) were collected from the Mississippi Sound near the offshore barrier islands using various gear. Otolith assessments provide estimated ages for 161 of those specimens (71 males, 78 females). Males and females ranged from 0 to 7 and 1 to 11 years of age, respectively, with a maximum age of 7 for males and 11 for females. Size and age at first maturity were 293mm and 1 year for males and 353mm and 3 years for females.
Female sheepshead are heavier at a given size than
males.
Otolith ring deposition occurs annually in late spring.
Sheepshead in Mississippi range from 0-6 years; age 4
fish were most common. There is a large variation in
size at age for both sexes.
Males are longer than females at a given age and reach a
larger maximum size.
Mississippi sheepshead appear to have a shorter life span
than those from South Carolina, Florida and Louisiana.
All males examined were sexually mature; females reach
100% sexual maturity by age 3.
Sheepshead spawn in Mississippi waters during March
and April.
There is no relationship between batch fecundity and
female size; mean batch fecundity is 219,100
eggs/female; mean relative fecundity is 214 eggs/g
ovary-free body weight.
Mississippi males and females reach sexual maturity at a
younger age than Louisiana and Florida sheepshead, and
Mississippi females are more fecund than Louisiana
females.
Additional data are necessary to confirm the above
conclusions.
Summary
Results and Discussion
Age and Growth. Female sheepshead ranged from 86 – 558 mm TL. Male sheepshead were similar in size, ranging from mm TL. Both sexes showed a significant, positive relationship between length and weight. Females tend to be heavier at a given size than males. Marginal increment analysis was conducted on otoliths from age 4 sheepshead (N=52) to validate the formation of annular rings. A minimal clear zone, suggesting recent deposition of a ring, was observed in May and August, although these were also months with greatest variability. Marginal increment analysis of age 3 (N=46) also showed a minimum in May. Additional data are necessary to verify annular formation of opaque bands in sheepshead otoliths, but these preliminary data suggest a ring is laid down once a year in May. Thus, we assume that age is equivalent to the number of rings on an otolith. Marginal increment analysis of sheepshead in Louisiana (Beckman et al. 1991) and South Carolina (Wenner 1996) indicated formation of a single annulus in the spring (April-May), similar to our findings.
Sheepshead ranged from 0 to 11 years, with age 4 fish most common in our samples. Mississippi sheepshead appear to be much younger than sheepshead from other areas, based on maximum ages of fish collected in South Carolina (26 years; Wenner 1996), Louisiana (20 years; Beckman et al. 1991) and Florida (13-16 years; Murphy and MacDonald 2000). There was a highly variable but significantly positive distribution of lengths at age for both sexes (p = 0.01, r2 = 0.50, males, r2 = 0.38, females; Figure 6). The large variability in size at age is common for sheepshead, and has been reported for South Carolina, Florida and Louisiana. Males were longer than females at a given age, and the vonBertalanffy growth equation predicts a larger maximum size for male sheepshead than females (Figure 6). These results are in contrast to findings from Louisiana (Beckman et al. 1991), where females were predicted to be larger than males, although estimated Lmax for males from both areas are the same. However, K values suggest that Mississippi females exhibit more rapid growth to an asymptotic size than males.
Sheepshead. Sheepshead, Archosargus probatocephalus, occurs in the coastal waters of the western Atlantic Ocean from Nova Scotia to Brazil, with the densest populations found in the Gulf of Mexico (citation here). This species supports an important winter recreational and commercial fishery in Mississippi’s coastal waters. Recreational catch has ranged from 22,815 to 250,031 fish harvested in 1999 and 1994, respectively (citation here). Mean annual catch over the last ten years has been 101,429 fish with mean annual harvest weight of 233,707 pounds. Mean size over the last ten years has been 14.3 inches TL.
Deegan (1990) did not include sheepshead as a species of preference targeted by fishermen in a Mississippi angler survey conducted during the early 1990's. This species is apparently caught and landed by fishermen but is probably considered as by-catch for most fishing trips. These fish are targeted by a small group of fishermen, especially during the winter time along inshore structures such as piers and bridge pilings. Sheepshead have specialized front teeth that allow them to feed on barnacles and other mollusks attached to submerged structures. These fish also eat a variety of crustaceans (Overstreet and Heard, 1982).
This species is generally under represented in fishery surveys along the Gulf coast, probably due to gear bias (LDWF, 2001). Inshore assessment of this species will require implementation of specific gear in specific areas. Wilson et al. (1989) has provided the only report along the northern Gulf coast for age and growth information on adult sheepshead using otoliths for ageing. There is a need for further investigation in determining the population age structure and growth of sheepshead in Mississippi waters for management purposes. Seasonal abundance and fecundity estimates will also be useful.
Introduction
Lmax = 479
K = 0.379
t0 = 0.581
Lmax = 540
K = 0.244
t0 = 0.572
Figure 4. Age-length relationships of sheepshead
Dec. 03 – Nov. 05
Field Sampling. Exploratory gill nets were conducted on a monthly basis. Area of sampling was determined by talks with local fishermen and previous hook and line success around bridge pilings and other known structures (Figure 1). This effort helped to establish the presence or absence of fish that could possibly not be susceptible to capture by the current gear employed and would not be observed in the gill net sets at particular locations. Hook-and-line sampling provided the majority of fish for laboratory analysis and could possibly be comparable to recreational catch. Environmental parameters measured on each sampling trip included surface water salinity (‰), temperature (°C) and dissolved oxygen (mg/L), and turbidity was estimated with a secchi disk.
Laboratory Analysis. Sheepshead from all available sources were labeled, immediately iced and returned to the laboratory for evaluation. Total length (TL, mm), standard length (SL,mm), total body weight (g) and sex were recorded for individual fish. Otoliths were removed from specimens for ageing purposes. Ovaries and/or testes were staged by gross observation (Table 1) and selected reproductive tissue was removed for further microscopic analysis. The reproductive tissue was prepared according to standard histological procedures.
Age/Growth – Sagittal otoliths were extracted from the heads of all specimens and thoughly cleaned of excess tissue and air-dried. One otolith from each fish was embedded in an epoxy resin and sectioned transversely at 0.3 mm with a Buehler Isomet low-speed saw. Otoliths were sectioned through the core along a transverse, dorsoventral plane. Sectioned otoliths were viewed under a dissecting microscope (20-40X magnification) using transmitted light. The authors were familiar with reading opaque bands on sectioned sagittae and counted the rings on otoliths as the opaque band between subsequent translucent zones. Ring counts were carried out “blind”, i.e. the readers did not know the size or sex of the sheepshead.
Materials and Methods
Reproduction. Sheepshead have a relatively short reproductive period. GSI values of both males and females are elevated from February – April, with a peak in March (Figure 7). This corresponds to previous reports of a February-April spawning season in Louisiana (Render and Wilson 1992) and spawning in March and April in Georgia (Music and Pafford 1984).
Histological evidence of sperm in the ducts was found December – May, although running ripe males were only captured in March, corresponding to the GSI peak (Figure 7). No immature males were observed in the sample; the smallest male examined histologically (293 mm TL, age 1) had spermatozoa in the testis. Nearly all Florida males are also sexually mature by age 1 (Murphy and MacDonald 2000). In contrast, Render and Wilson (1992) found male sheepshead in Louisiana did not reach 100% maturity until age 4.
Females in spawning condition (FOM class) were only found in March and April, corresponding to the GSI peak (Figure 7). Regressed females were found during all months except the time of elevated GSI values (February – April). All females <294 mm TL were immature, and all females >353 mm TL were sexually mature. The majority of age 1 (67%) and age 2 (75%) females were immature; all females ages 3-6 were sexually mature. In contrast, female sheepshead in Louisiana and Florida did not reach 100% sexual maturity until age 5 (Render and Wilson 1992; Murphy and MacDonald 2000). There are insufficient data to accurately determine size or age at 50% maturity for females in Mississippi. All hydrated females were taken from the offshore barrier islands, supporting previous suggestions that sheepshead primarily spawn offshore (Music and Pafford 1984; Render and Wilson 1992)
There was no relationship between fish size and batch fecundity. Batch fecundity estimates of 7 fish ranged from 144,010 – 363,321 eggs, with mean batch fecundity of 219,100 ± 36,700 eggs/female. This is greater than batch fecundity reported for 20 Louisiana females, which ranged from 1,100 – 250,000 eggs with a mean of 47,000 (Render and Wilson 1992). Mean relative fecundity of Mississippi fish is 214 ± 42 eggs/g ovary-free body weight.
Figure 5. Monthly GSI values (mean ± SE) of male and female sheepshead
Beckman, D.W., A.L. Stanley, J.H. Render and C.A. Wilson Age and growth-rate estimation of sheepshead Archosargus probatocephalus in Louisiana waters using otoliths. Fish. Bull 89:1-8.
Murphy, M.D. and T.C. MacDonald An assessment of the status of sheepshead in Florida waters through Florida Marine Research Institute, St. Petersburg, FL. 50 p.
Music, J.L., Jr. and J.M. Pafford Population dynamics and life history aspects of major marine sportfishes in Georgia’s coastal waters. GA Dept. Nat. Res., Coastal Res. Div., Coastal Fish Section, Final Report. Study IV Federal Aid Project F p.
Render, J.H. and C.A. Wilson Reproductive biology of sheepshead in the northern Gulf of Mexico. Trans. Am. Fish. Soc. 121:
Wenner, C.A Age and growth of sheepshead, Archosargus probatocephalus, from Wouth Carolina waters with some preliminary management concepts. SC Dept. Nat. Res. Final Report, job 6 of F p.
Literature Cited
Acknowledgements
We would like to thank the Department of Marine resouces, along
with Gulf Coast Geospatial Center, Jim Franks, Eric Hoffmayer,
PhD, Read the Gulf Coast Research lab University
Of Southern Mississippi.