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Lepisosteus oculatus


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Summary

The upper Barataria Estuary is a cypress-tupelo floodplain that once received freshwater input from the Mississippi River. Levee construction and the closing of distributaries now prevent an annual river-driven floodpulse from entering this system. The loss of an annual floodpulse can have negative impacts on the reproductive potential of fish species such as spotted gar Lepisosteus oculatus and bowfin Amia calva that rely on an inundated floodplain for spawning. In 2006, macroscopic examination of bowfin ovaries revealed 96% egg atresia in the upper Barataria Estuary. However, in 2006, based on gonadosomatic indices (GSI), the gizzard shad Dorosoma cepedianum population in this system spawned from late March through May. Data are needed to better understand gonad development in spotted gar in the upper Barataria Estuary. The purpose of this study is to histologically examine gonadal reproductive stages of spotted gar and to determine GSI, fecundity, age distributions, and size distributions of spotted gar in the upper Barataria Estuary. Spotted gar will be collected each month from October 2006 to October 2007. Standard histological techniques will be used to assess gonadal stages of spotted gar. Whole preserved gonads will be used to estimate fecundity, and weights of whole gonads will be used to determine GSI of spotted gar. Length, girth, weight, and age (determined from otoliths) will be used to determine age and size distributions of spotted gar. Information from this study will be useful in understanding gonadal maturation and life history of spotted gar in the upper Barataria Estuary. Results from this study will contribute to long-term evaluation of fish populations in the upper Barataria Estuary. This large-scale study can lead to a better understanding of the effects of the loss of an annual river-driven floodpulse on the reproductive success of fish in this system.



Introduction

The upper Barataria Estuary is a cypress-tupelo swamp consisting of Grand Bayou, Bayou Citamon, Bayou Chevreuil, the St. James Canal, and Lac Des Allemands (Figure 1). This system once received an annual floodpulse from the Mississippi River; however, through levee construction and closing of distributaries, the upper Barataria Estuary is no longer annually inundated by a predictable floodpulse. Presently, inundation of the upper Barataria Estuary results from unpredictable local precipitation. The timing and duration of a river-driven floodpulse corresponds with the spawning period of many fish species in large-river floodplains (Junk et al. 1989). During spawning seasons, many species of fish (e.g.; spotted gar Lepisosteus oculatus and bowfin Amia calva) move onto inundated floodplains to feed and spawn in the shallow vegetated waters (Snedden et al. 1999; Bonvillain 2006; Davis 2006). Therefore, the lack of an annual river-driven floodpulse could have negative impacts on the reproductive success of floodplain-dependent fish species. When floodplain-dependent fish are denied access to spawning habitat, the reproductive output of the populations could considerably decrease. Additionally, when the floodpulse is absent, primary and secondary production decrease in floodplain systems, reducing food availability for fish species that forage on the inundated floodplain (Bayley 1995). The resulting reduced juvenile survival also contributes to a reduction in the overall reproductive health of these floodplain-dependent fish species.

Spotted gar are members of the ancient family Lepisosteidae and can be found in bayous, lakes, and backwater floodplains (Douglas 1974; Snedden et al. 1999; Bonvillain 2006). The range of spotted gar includes the Great Lakes to the Gulf of Mexico and from central Texas to western Florida (Douglas 1974). Spotted gar are commonly found in floodplain ecosystems such as the upper Barataria Estuary (Suttkus 1963; Snedden et al. 1999; Bonvillain 2006). In a study

(a)

Lac Des Allemands

Bayou Chevreuil

Mississippi River

St. James Canal

Bayou Citamon

Grand Bayou
(b)

Figure 1. The upper Barataria Estuary in Louisiana (a) consists of Bayou Citamon, Bayou Chevreuil, Grand Bayou, and the St. James Canal, all of which drain in an east-southeast direction into Lac Des Allemands (b).
by Davis (2006), which targeted bowfin in the upper Barataria Estuary, 38% of the total fish species caught were spotted gar. Spotted gar is a useful species model in this study because it represents the ancient fishes, particularly holosteans. Additionally, spotted gar are valuable predatory components of floodplain ecosystems through their cycling of nutrients and control of lower trophic levels species (Scarnecchia 1992).

The gars were once regarded as nuisance predators of game fish (Gowanloch 1939; Gowanloch 1940; Suttkus 1963). Therefore, there has been a lack of reproductive data on these species, which would aid in formulating management plans. Gars, however, are enjoyed as game and food fish in Louisiana (Sutton 1998), and there have been movements to properly manage these species in the southern United States (Scarnecchia 1992; Todd 2005). In 2003, Louisiana commercial fisheries landings for gars (spotted gar, longnose gar Lepisosteus osseus, shortnose gar Lepisosteus platostomus, and alligator gar Atractosteus spatula) were greater than $515,000 (LDWF 2003).

In 2006, macroscopic examination of bowfin ovaries revealed egg atresia in 96% of females sampled from February 2006 to May 2006 (N=136; Davis 2006). Apparently, the majority of bowfin did not spawn in the upper Barataria Estuary. However, based on gonadosomatic index (GSI), the gizzard shad Dorosoma cepedianum population in the upper Barataria Estuary spawned from late March through May 2006 (Fontenot 2006). Additionally, GSI, age distributions, and size distributions have been determined for the bowfin (Davis 2006) and gizzard shad (Fontenot 2006) populations in the upper Barataria Estuary. Unlike the bowfin and gizzard shad populations, there is little data on the life history and reproduction of spotted gar in the upper Barataria Estuary. Therefore, a detailed analysis is needed to better understand reproductive stages of spotted gar, especially with the loss of an annual floodpulse. We propose to use histological analysis of ovaries to support macroscopic observation of ovarian development and potential egg atresia if spawning does not occur. Additionally, fecundity, GSI, age distributions, and size distributions will be determined for spotted gar in the upper Barataria Estuary.

Histology techniques will be used to identify gonadal stages on a microscopic level (e.g.; immature, early maturation, mid maturation, late maturation, spawning, spent, and recovering; Brown-Peterson 2006). Immature ovaries are classified by a thin ovarian membrane and primary growth oocytes (chromatin nucleolar and perinucleolar oocytes; Brown-Peterson 2006), which possess follicle cells, multiple nucleoi, and perhaps lipid vacuoles and the vitelline envelope in the later stages (West 1990). Brown-Peterson’s (2006) ovarian maturation categories include West’s (1990) stages of cortical alveoli (CA) and yolk formation, and these stages involve the final formation of lipid vacuoles and the vitelline envelope. Late ovarian maturation is also apparent with the presence of final oocyte maturation (FOM; Brown-Peterson 2006), which is the presence of unovulated, vitellogenic, hydrated oocytes (Lowerre-Barbieri and Barbieri 2006). The spawning stage is categorized by the presence post-ovulatory follicles (POFs; Brown-Peterson 2006), which are remnant follicles of spawned oocytes (Lowerre-Barbieri and Barbieri 2006). Spawning ovaries may also include FOM, atretic oocytes, CA oocytes, and vitellogenic oocytes (Brown-Peterson 2006). Spent ovaries possess atretic oocytes, POFs, and some CA oocytes, and regressed ovaries contain atretic and primary growth oocytes (Brown-Peterson 2006).

In males, Brown-Peterson (2006) classifies immature testes by the presence of primary spermatogonia, small lobules, and the absence of a lumen. The early and mid testicular maturation stages are classified by the presence of a continuous germinal epithelium (GE) and all stages of spermatogenesis, including spermatozoa (Brown-Peterson 2006). Late maturation involves the presence of a discontinuous GE, and spermatozoa are the dominant cells in this class (Brown-Peterson 2006). Males capable of spawning can have characteristics of all stages of maturation and the spent stage, which is classified by a discontinuous GE and all stages of spermatogenesis (Brown-Peterson 2006). Regressed testes are classified by a continuous GE and residual spermatozoa (Brown-Peterson 2006).

By histologically examining these gonadal stages, more information can be obtained on the reproduction and spawning sequences of spotted gar in the upper Barataria Estuary. If our analysis reveals reproductive failure of spotted gar in this system, perhaps further research can link the failure to the loss of the annual floodpulse.



Goal and Objectives

The goal of this project is to describe reproductive stages and to determine the population structure of spotted gar in the upper Barataria Estuary. This study will include histological analysis of gonadal stages and assessment of life history characteristics of the spotted gar population in the upper Barataria Estuary. The specific objectives of this project include the following:



  1. Document and quantify reproductive stages of spotted gar for one year in the upper Barataria Estuary using standard histological techniques.

  2. Determine age and size distributions of spotted gar in the upper Barataria Estuary.

  3. Quantify age-specific fecundity of female spotted gar in the upper Barataria Estuary.

  4. Quantify sex-specific, seasonal changes in GSI of spotted gar in the upper Barataria Estuary.

Methods

Field Sampling and Laboratory Collection

Spotted gar will be sampled each month from October 2006 to October 2007, in the upper Barataria Estuary using monofilament gill nets. At each sampling site, dissolved oxygen (DO; mg/L), temperature (ºC), specific conductance (µS), salinity (ppt), and Secchi disk depth (cm) will be measured in order to describe the sampling habitat, which frequently experiences fluctuations in these parameters (Estay 2007). Fish will be kept alive in an ice chest with water until being processed in the Bayousphere Research Laboratory at Nicholls State University (NSU). Total length (TL; mm) and pre-pelvic girth (mm) will be measured for each individual. Body weight (0.1 g) and left and right gonad weights (0.01 g) will also be measured. Photographs will be taken of gonads to macroscopically document maturation. Sex determination will be based on gonad observation (Ferrara and Irwin 2001).

Up to 360 gonad histology samples will be preserved in 10% neutral buffered formalin (NBF). Approximately 15 male and 15 female histological samples per month will be sent to the Louisiana State University (LSU) School of Veterinary Medicine Department of Pathobiological Sciences for embedding in paraffin and sectioning and staining with hematoxylin and eosin (H and E). The remainder of the spotted gar gonads will be preserved in 10% non-buffered formalin. Ten fresh eggs, prior to preservation, will be randomly selected from the ovaries of each female spotted gar to measure egg diameters (0.1 mm) using digital calipers. For each spotted gar, otoliths will be removed, washed, cleaned, and placed in labeled plastic vials for age determination (Ferrara 2001).

Analyses

Histology slides will be examined with compound light microscopy to document the succession of reproductive stages throughout one year. Histology samples will be classified according to the system derived by Brown-Peterson (2006) with the following categories: immature, early maturation, mid maturation, late maturation, spawning, spent, and regressed. Weights of whole gonads will be used to determine GSI, which will be calculated according to Snyder (1983):

GSI = (gonad weight) / (total body weight) x 100.

Whole preserved gonads will be used to estimate fecundity of spotted gar. Fecundity will be determined by counting a 10% (by weight) subsample of each ovary (Ladonski 1998). Total number of eggs in each ovary will be extrapolated by multiplying the number of eggs in the 10% subsample by 10. Each month, whole counts of each ovary will be determined for a minimum of two randomly selected female spotted gar. A paired t-test (α=0.05) will be performed to determine if there is a difference in egg estimation between the 10% subsample count method versus the whole ovary count method. Water quality data will also be used to determine any potential effects on gonad development (e.g.; regressions of water temperature versus egg diameter/reproductive stages). Length, girth, weight, and age (determined by otoliths) will be used to determine sex-specific age and size distributions of spotted gar. Additional spotted gar that will not be used for histological analysis will be processed for life history analysis (e.g.; GSI, fecundity, age distribution, and size distribution).


Preliminary Data

To date, 401 spotted gar have been collected. Measurements of TL, girth, weight, sex, and left and right gonad weights have been recorded for each individual (Table 1). In the upper Barataria Estuary, a one-way ANOVA test revealed that females are longer than males (P<0.0001) and are heavier than males (P<0.0001). The mean egg diameter for 99 female spotted gar was 2.63±0.16 mm. Histology samples have been collected for 181 spotted gar, and 27 of the spotted gar samples have been processed by the LSU School of Veterinary Medicine Department of Pathiobiological Sciences. To date, no analysis of gonad histology has been performed. Weight-length relationships have been determined for males (Figure 2; Weight=0.000002(Total length3.1498)) and females (Figure 3; Weight=0.000001(Total length3.2011)). Mean GSI values per sample date for males ranged from 0.6 to 2.1 and were highest in March 2007 (Figure 4). Mean GSI values per sample date for females ranged from 5.9 to 14.1 and were highest in April 2007 (Figure 5). To date, mean fecundity for 8 spotted gar (mean total length: 571±23 mm) has been determined to be 5,261±2,075 eggs using the 10% count method. Additionally, otoliths from 396 spotted gar have been collected for age determination.



Table 1. Mean (±SD) total length, girth, weight, left and right gonad weights, and sex percentages for male and female spotted gar (N=401) collected from October 2006 to May 2007, in the upper Barataria Estuary.


Figure 2. Weight-length relationship for male spotted gar collected from October 2006 to May 2007, in the upper Barataria Estuary.

Figure 3. Weight-length relationship for female spotted gar collected from October 2006 to May 2007, in the upper Barataria Estuary.
Figure 4. Mean (±SD) gonadosomatic index for male spotted gar (N=191) collected from October 2006 through May 2007, in the upper Barataria Estuary. No individuals were collected in January 2007.
Figure 5. Mean (±SD) gonadosomatic index for female spotted gar (N=210) collected from October 2006 through May 2007, in the upper Barataria Estuary. No individuals were collected in January 2007.
Significance of Research

This study will describe reproductive stages of spotted gar in an altered floodplain system. Results will contribute to a long-term evaluation of the upper Barataria Estuary. Information from this large-scale study will be useful to managers in making decisions for future freshwater diversions in the Barataria Estuary. If our long-term results demonstrate a negative impact on the reproductive potential of resident fish species due to the loss of the floodpulse, perhaps Mississippi River water can be diverted into the upper reaches of the estuary through freshwater diversion outlets. An increase in freshwater would allow for seasonal inundation of the floodplain in the upper Barataria Estuary, increasing the available spawning habitat for species such as spotted gar and bowfin.

The reproductive potential and life history of spotted gar throughout its range are poorly understood. Therefore, this study will provide data specific to the population of spotted gar in the upper Barataria Estuary. Because the upper Barataria Estuary is no longer annually inundated by a river-driven floodpulse, the potential exists for negative impacts on the reproductive potential of spotted gar in this system. Because spotted gar are also top level predators in large-river floodplains, there is also a potential for negative impacts on organisms dependent upon the spotted gar (e.g.; competitors of prey species of spotted gar). Understanding life history characteristics of spotted gar will help in establishing regulations (e.g.; regulating size and bag limits and fishing seasons) for this species if these negative impacts occur, particularly in regions where spotted gar are not very common (e.g.; edges of its range). Overall, this study will be useful in understanding the fauna of the upper Barataria Estuary. In combination with other projects in the Bayousphere Research Laboratory at NSU, this project will lead to a better understanding of the overall system health of the upper Barataria Estuary.

Literature Cited

Bayley, P. B. 1995. Understanding large river floodplain ecosystems. Bioscience 45(3):153-158.


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