Comparative growth of pinfish in field mesocosms across marsh, oyster, and soft-bottom habitat types in a Mississippi estuary Virginia Shervette and Fran Gelwick Department of Wildlife and Fisheries Sciences Texas A&M University
WHY? Pritchard definition of estuary: Estuaries are defined as semienclosed coastal bodies of water, which have free connection with the open sea, and within which seawater is measurably diluted with freshwater from land drainage Estuaries are extremely susceptible to human disturbances Understanding how species utilize estuarine habitats is critical for future conservation efforts in estuaries
Nursery-role hypothesis Nursery habitat is one that recruits more individuals per unit area to the adult population than other habitats containing juveniles of the same species A combination of density, growth, survival of juveniles within a delineated nursery habitat type, and the successful movement of juveniles from this nursery habitat to an adult habitat must be greater when compared to other juvenile habitat types Different habitat types offer varying degrees of complexity to shelter juveniles from predation Variations in quantity and quality of food resources across habitat types affect the rate of development, which has consequences on survival Juvenile pinfish are found across simple and complex habitat types including seagrasses, marsh edge, oyster reefs, and soft bottom making them model organisms for the study of the nursery role hypothesis through density, growth, and survival research
Assessment of Essential Fish Habitats as nurseries for economically important fishes Focus of our research in Grand Bay NERR is to elucidate the differences in nursery value of several juvenile fish habitats for individual species such as pinfish (Lagodon rhombiodes) through assessment of density, growth, and survival in available habitat types July 2003, we conducted a pilot study to elucidate the differences in growth of pinfish across three habitat types: marsh, soft-bottom, and oyster rubble reef
Grand Bay National Estuarine Research Reserve Grand Bay is bordered on the west by the heavily industrialized Pascagoula estuary and on the east by another heavily industrialized estuary, Mobile Bay
Materials and Methods: Why growth? Expeditious growth possibly gives juvenile fishes ecological advantages by increasing survivorship through the reduction of time a juvenile fish spends in size classes more vulnerable to predation Shallow estuarine habitat provides juvenile fishes with abundant food sources imperative to rapid growth Several habitat types are available for juvenile fishes in Grand Bay Overall objective for pilot study is to determine the feasibility of experiment assessing the growth rate among various estuarine habitat types for juveniles of several fish species
Growth field mesocosms Quantify growth within each habitat type Field enclosures restrict fishes to a habitat type and exclude predators, but allow access to the bottom substrate for foraging Used successfully to measure fish growth rates in a variety of habitats with a variety of fish species Circular enclosures, 0.6 m radius and 1.0 m tall (0.283 m2) consisted of a polypropelene barrel with the bottom and top removed and windows cut out covered with 3 mm mesh nylon netting on top and sides Barrels pushed into sediment at least 15 cm and a 20 cm lip extending from the ground to the bottom edge of the windows allowed water to collect for lowtide to maintain fishes
Three replicates of mesocosm in each habitat type (mud, oyster, marsh) Collected initial and final DO, salinity, and temperature data for each mesocosm with YSI
Used numbered grid placed over map of area and random numbers table to determine placement of mesocosms
Enclosures were swept repeatedly in order to remove potential competitors and predators The enclosures were anchored from the outside with metal stakes Each growth mesocosms stocked with three pinfish collect from nearby habitats with a seine
To follow the growth of individuals, each of the fish used within the enclosures were marked with a distinctive pelvic fin clipping after first being anesthetized with an appropriate dose of MS-222 (in order to reduce handling stress) Fish were then measured (TL and SL) to the 0.01 mm Fish released immediately into mesocosms and observed until exhibited normal behavior
Each mesocosm had a mesh lid stapled onto it
High tide
Fish remained in mesocosms for 10 days Checked site daily
At the end of the growth period, fishes were collected from enclosures by using dip nets Immediately upon collection, fish were anesthetized with MS-222, identified according to fin clippings, measured, and then placed in 70% ethanol Determined growth rates of individual fish by subtracting the final standard length measurements for each individual fish from their initial length and dividing by 10 (days)
Statistical analysis and caveats The null hypothesis that we tested was: There is no difference in average growth of individuals among habitat types. Used one-way ANOVA with habitat type as the factor and average growth rate as the response variable Did not violate assumptions of normality or homogeneity of variance Artifacts of design: For experiments conducted within enclosures, as long as the effects of the enclosures are kept constant across all experimental treatments then the effects of artifacts are also held constant. The results from the enclosure growth experiments address the potential growth and comparative habitat value of the habitat types. Since we are not trying to assess absolute growth of fishes within the different habitat types, and only assess relative growth and the enclosures used within each habitat type are the same size, composition, and duration, artifacts of sampling design should not be a problem (Peterson and Black 1994). A possible criticism concerning the use of enclosures is that experimental fish may deplete food sources within the experimental time period. In response, we would like to point out that Gallagher (2001) found that enclosure structures used in soft sediment environments had little affect on overall density and community structure of potential prey. She also found that inclusion/exclusion of predatory pinfish had no significant effect on the immigration and emigration rates of potential prey species within field enclosures.
Mean growth (mm/day) 1 0.8 0.6 0.4 0.2 0-0.2 Results mud oyster marsh -0.4 Habitat type Growth rates of pinfish in the Spartina marsh and potentially oyster habitat types were significantly higher than the soft bottom habitat type (F=7.436, p<0.005; Bonferroni post hoc: marsh > mud: p < 0.05) Pinfish grew an average of 0.52 mm/day (SD = 0.36) in the marsh habitat type, 0.30 mm/day (SD = 0.19) in the oyster habitat type, and -0.05 mm/day (SD = 0.24) in the soft bottom habitat type
Discussion Results suggest that growth potential for pinfish is greater in structured habitat (marsh and maybe oyster) Supported by habitat differences in food abundance in structured habitat Is this true for all sites and estuaries? Consider pilot study a success and plan on conducting it again on a much greater scale Four sampling areas, 3 4 habitat types (addition of seagrass), more species (4 5), three seasons
Extent of inter-habitat movement affects utility of growth rates Settle-and-stay hypothesis predicts that vulnerability to predation reduces movement of fishes once they have settled into a habitat Currently testing the practicality of settle-and-stay by morphological analysis of pinfish (and some sciaenid species) collected from various habitat types (Cliff Ruehl and Thomas DeWitt)
Now what? Growth study at larger scale (2 estuaries: Grand Bay and Weeks Bay, Alabama) Assessing density and will assess survival Creation of model with management implications
MUCH THANKS GO TO: Stuart Ralston Grand Bay NERR TCWC, DeWitt lab, and Winemiller lab for ethanol barrels Committee members: Tom Minello, Steve Davis, Mike Speed, and Bill Grant Wildlife and Fisheries Sciences students: Steve, David, Jenny, Cliff, Natalie FUNDING PROVIDED BY: NOAA/NERR Graduate Research Fellowship, Graduate Women in Science Fellowship, International Women s Fishing Association Texas Water Resource Institute Mill s Scholarship