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Fisheries & Ecology
Affiliated research principals:
Dr. G. Joan Holt
Scott Holt
Events in the early life of fish (egg, larval and early juvenile stages) are crucial factors in the success or
failure of fish populations. They have developed a wide range of biological adaptations to diverse habitats;
some have adaptated to specific habitats while others are broad generalist, making use of a variety of habitats.
How fish come to populate these various habitats, and how habitat quality and quantity affect the well being of
individual fish and greater populations are a central focus of FAML’s fisheries and ecology research.
Our research projects are integrative, employing both field and laboratory approaches to investigate questions
regarding fish ecology. Extensive sampling of larval, juvenile, and adult fish as well as field-based experiments
are conducted in estuarine and offshore environments. Laboratory work includes analyses of field samples and
laboratory based experiments. A variety of analytical techniques are used in our research, including satellite tag
tracking, ichthyoplankton identification, image analysis, mass spectrometry, and various biochemical assays.
Our work appears in peer-reviewed journals, as well as conferences and events that span the government, academic,
conservation, and resource stakeholder communities.
Fisheries & Ecology Research Areas:
Tarpon migration and ecology [Show Details]
Tarpon once thrived off the coast of Texas (FAML’s home town of Port Aransas used to be called “Tarpon, Texas”). In the mid-1950’s fishermen observed a catastrophic crash in tarpon populations.
To this day, we do not know what factors contributed to the decline of this magnificent sportfish. FAML has received funding from several institutions to investigate migratory patterns of tarpon
both on the micro and macro levels. We will attach satellite pop-off tags to several adult specimens to reveal clues as to how and why tarpon migrate where they do along the northwestern Gulf
of Mexico. Our goal is to gain insight into how this population reproduces, and provide both U.S. and Mexican fishing communities with recommendations on how to protect and restore this species
to former levels.
Catch-and-release mortality rates in red snapper [Show Details]
Red snapper are prized sport and commercial food fish everywhere they are found. Intense fishing pressure has caused the species to become one of conservation concern. Strict size limits require
sport fishermen to release specimens fewer than 16 inches in length. The fact that these fish are initially hooked at significant depths causes gases in their bodies to expand as they are brought
to the surface, causing significant stress and injury risks that may result in mortality after release. The Coastal Conservation Association has generously provided funding to investigate mortality
rates from hook-and-line fishing. This research will provide fisheries managers valuable information to base decisions affecting size and catch limits for the species.
Early life-history of billfish in the Gulf of Mexico [Show Details]
Billfish (such as blue marlin (Makaira nigricans), white marlin (Tetrapturus albidus), and sailfish (Istiophorus platypterus) are found in the Gulf of Mexico, where they play an important
role as apex predators and are a valuable recreational fishery. Over the past three decades billfish populations have been in decline - most likely due to by-catch mortality from commercial longline fishing.
The purpose of our project is to identify areas of the Gulf of Mexico with high densities of larval billfish. Very little information is known about the early life history of these fish,
which is vital to producing sound scientific management policies to maintain healthy populations. Billfish from ichthyoplankton sampling will be photographed, identified using DNA, measured,
and their age determined by extracting the otoliths. Gut contents of each fish will be analyzed to determine the diet of larvae at specific stages. In addition, we will analyze the RNA:DNA ratios to determine
if specific sampling sites have larvae with higher growth rates than other sites.
Our goal is to locate the spawning grounds of billfish and categorize the characteristics of essential fish habitat for the larvae. This information can be used to identify other spawning areas and essential
fish habitat in the Gulf of Mexico. Fisheries biologists can use this data to make management decisions that ensure populations of marlin, sailfish, and spearfish will be healthy for years to come.
Pertinent references:
Luthy, S.A. (2004). Billfish Larvae of the Straits of Florida. PhD Dissertation. University of Miami, Coral Gables, Florida, 112 pgs.
Serafy, J.E., Cowen, R.K., Paris, C.B., Capo, T.R., Luthy, S.A. (2003). Evidence of blue marlin, Makaira nigricans, spawning in the vicinity of Exuma Sound, Bahamas. Marine and Freshwater Research 54, 299-306.
Sponaugle, S., Denit, K.L., Luthy, S.A., Serafy, J.E., Cowen, R.K. (2005). Growth variation in larval Makaira nigricans. Journal of Fish Biology (2005) 66, 822-835.
Distribution and transport of red drum larvae and eggs [Show Details]
Recruitment variability in many marine fish species is often attributed to physical and biological processes acting upon the early life stages. Settlement, or the transition between the
planktonic and demersal habitats, is an important component of recruitment in species inhabiting many ecological systems.
Little is known about the settlement dynamics of red drum (Sciaenops ocellatus), an important recreational species found in the Gulf of Mexico and southeastern Atlantic coast. Previous studies
indicate that the presence of planktonic red drum larvae in the Aransas Pass tidal inlet in South Texas is associated with coastal waters entering the estuarine system during flood tide.
We are evaluating the extent of coupling between the abundance of potential settlers entering the estuarine system through the tidal pass (larval supply) and subsequent settlement to extensive
seagrass nursery habitat. In addition, we are assessing the spatial coherence of settlement to different areas of the estuarine system to determine if large-scale physical processes are driving the observed settlement patterns.
The results of this study so far have been integrated with a hydrodynamic particle transport model being developed at Texas A&M University to acquire a cohesive understanding of the processes
leading to successful recruitment of red drum and similar estuarine dependent species.
Pertinent references:
Holt, S.A., C.L. Kitting, C.R. Arnold (1983) Distribution of young red drums among different seagrass meadows. Transactions of the American Fisheries Society. 112:267-271.
Holt, S.A., G.J. Holt and C.R. Arnold (1989) Tidal stream transport of larval fishes into non-stratified estuaries. Rapp. P.-v. Reun. Cons. int. Explor. Mer. 191:100-104.
Rooker, J.R., S.A. Holt, M.A. Soto and G.J. Holt (1998) Postsettlement patterns of habitat use of Sciaenid fishes in subtropical seagrass meadows. Estuaries. 21:318-327.
Stable isotope ratios as tracers of larval settlement [Show Details]
Many marine species exhibit a life cycle characterized by a pelagic larval stage followed by transport into estuaries and settlement to nursery habitat. Identifying the factors controlling
the population regulation and recruitment of estuarine dependent species requires a thorough understanding of the fine-scale temporal dynamics of settlement. We are studying the settlement
dynamics of red drum (Sciaenops ocellatus). Using them as a model estuarine-dependent species, we’ve analyzed larval fish tissues to trace the settlement of individual larvae from
coastal waters to nursery habitat. Since animals reflect the isotopic composition of their food sources, differences in food web structure should translate into dissimilarities in the
isotopic composition of newly settled larvae and post-settlement early juveniles. By adequately characterizing the isotopic composition of pre- and post-settlement red drum we are working
to identify new settlers on an individual basis.
To track the settlement history of individual larvae, we have developed an individual-based model that can estimate size at settlement and time since settlement. The model is based on:
1) Characterization of the d13C of the larvae during the planktonic stage
2) The d13C of post-settlement early juveniles collected in the nursery habitat
3) The contribution of growth and metabolic turnover to the rate of isotopic shift
At the population level, the utilization of stable isotope ratios as tracers of the settlement history of individual larvae may be utilized to:
- Relate settlement events to physical forcing factors that drive the transport and delivery of larvae to estuarine nursery habitat
- Examine the temporal coupling between the abundance of larvae in the plankton and subsequent settlement
- Differentiate settlement from movement within the nursery habitat
- Identify critical settlement habitat
Pertinent references:
B. Fry and C.R. Arnold (1982) Rapid 13C/12C turnover during growth of brown shrimp. Oecologia. 54:200-204.
S.Z. Herzka and G.J. Holt (2000) Changes in isotopic composition of red drum (Sciaenops ocellatus) larvae in response to dietary shifts: potential applications to settlement studies. Canadian Journal of Fisheries and Aquatic Sciences. 57:137-147.
Settlement dynamics of red drum larvae [Show Details]
Red drum larvae are wide spread in the Gulf waters along the immediate coastline and their successful immigration to estuarine nursery grounds is strongly influenced by coastal currents.
Resource managers and permitting agencies have identified several critical data gaps regarding larval red drum that still present substantial impediments to the evaluation of anthropogenic
impacts on red drum populations. Among these are the vertical distribution of red drum larvae within the water column, the depth distribution of red drum larvae and the distribution of red
drum eggs. Resource managers have also cited the need for improved estimates of growth rates and, particularly, mortality rates for red drum. To help address these issues we will carry out
the following field investigations:
Determine the vertical distribution of red drum larvae in offshore waters
Determine the offshore distribution of red drum larvae through sampling transects running perpendicular to the shoreline
Data from both investigations will be used to improve our estimates of growth and mortality rates in red drum larvae. We will follow a water mass known to contain a relatively high density
of red drum larvae and measure vertical distribution of the larvae on an hourly basis over several days. These data will provide an unprecedented measure of diel vertical movement for red
drum larvae. The experiment will be repeated several times over the three year study period to ensure adequate samples size and evaluate temporal variability.
Many estuarine-dependent fish species utilize estuaries as nursery habitat while spawning in other areas. Due to the small size and limited mobility of larvae, successful transport from
spawning sites to nursery habitats is largely dependent on physical processes, yet this is one of the least understood components of the early-life history of these species.
We have used a Geographic Information System (GIS) to visualize the abundance and distribution of larval red drum in the coastal region, and their relationship to oceanographic characteristics
of the study region. Each of these may constitute determining factors in the magnitude of settlement to local nursery habitat. The results of this study thus far have been integrated with a
hydrodynamic particle transport model being developed at Texas A&M University to develop a cohesive knowledge of the physical mechanism driving distribution and movement of larvae in nearshore
regions as well as the transport of these larvae through tidal inlets and into nursery habitat in the local bays. An understanding of transport mechanisms and the environmental conditions belying
favorable transport conditions will go a long way towards explaining recruitment variability in marine fish populations.
Biochemical indices of larval condition [Show Details]
Mortality is exceptionally high (up to 99%) during the planktonic stage of many marine fish. Starvation and predation are major causal factors, but the proportion of mortality attributed to each is not well known.
It is possible that starvation may directly (starvation induced mortality) and indirectly (increased predation susceptibility, buoyancy changes, etc.) lead to substantial reductions in the abundance of larvae that
survive until the time of settlement, and eventual recruitment into the adult population.
The difficulty in assessing the role of these factors has led to a search for new techniques and approaches to evaluate them. We are using biochemical constituents such as digestive enzymes, RNA/DNA, and lipids as indicators
of nutritional condition in larval red drum (Sciaenops ocellatus). Variations in the amount of RNA/DNA in the body or enzyme present in the larval digestive system occur in response to feeding activity and success. Poor feeding
success along with starvation/poor nutritional condition are evidenced by reduced concentrations of RNA/DNA and chymotrypsin.
Pertinent references:
Applebaum, S.L. and G.J. Holt. 2003. The digestive protease, chymotrypsin, as an indicator of nutritional condition in larval red drum (Sciaenops ocellatus). Marine Biology.
Rooker, J.R., G.J. Holt, and S.A. Holt. 1997. Condition of larval and juvenile red drum (Sciaenops ocellatus) from estuarine nursery habitats. Marine Biology 127:387-394.
Immigration and settlement of southern flounder larvae [Show Details]
The southern flounder (Paralichthys lethostigma) population along the Gulf coast has been in a state of decline while most fisheries species in Texas waters are holding steady, or even increasing. Reasons for the decline
are unknown, but may be attributed to several factors including overfishing (from both the directed fishery and bycatch from bay shrimping operations), declines in quality and quantity of nursery habitat, and/or decreases
in survival and successful transport of larvae from offshore spawning grounds to estuarine settlement sites. Very little is also known about the early life history or juvenile habitat requirements of southern flounder
in the Gulf of Mexico region. In an effort to fill this knowledge gap, we are conducting studies to identify which life stages are most critical concerning recruitment variability and whether anthropogenic impacts could affect recruitment success.
The goals of this study are:
- Determine the temporal pattern of larval southern flounder immigration through the Aransas Pass and the estuarine distribution of those planktonic larvae
- Identify primary nursery sites of southern flounder in both the Aransas and Galveston estuaries and assess patterns of habitat use
- Estimate growth and recruitment potential of southern flounder residing in different habitats within the Aransas and Galveston estuaries
This project utilizes two approaches to address these objectives. The first phase, carried out in the Aransas Pass inlet and the Aransas/Copano estuary, looks at immigration of planktonic larvae into the estuary, thus
providing data on timing of immigration and identification of potential settlement sites. Age and growth rates derived from otolith analysis provide information on environmental effects on growth and allow us to compare
the relationship between early growth rates and survival.
The second phase of the study entails sampling post-settlement juveniles to discern critical nursery habitats within the estuaries. These collections are from mirrored studies in Galveston and Aransas/Copano Bays,
providing a comparison of estuaries that differ in freshwater inflow regimes. This allows us to determine whether the characteristics of quality flounder nursery habitats differ among estuaries.
Spatial and temporal variability in settlement of red drum larvae [Show Details]
Organisms are often patchily distributed in nature. While many studies have examined differences in abundance of organisms between distinct habitat types, we have less information about the extent and causes of variability
in abundance within apparently homogenous habitat. This is a particularly important issue in marine fisheries ecology because there is typically extremely high mortality during the early life stages. The number of fish
surviving through the larval and juvenile periods may be the best predictor of year class strength. Without accurate estimates of how many individuals survive this period of high mortality, there is a lot of uncertainty
in predictions of year class strength.
We are working to understand the extent of variability in abundance of red drum (Sciaenops ocellatus) larvae over ecologically important time and space scales. Adult red drum spawn near shore and the larvae enter
estuaries by passing through tidal inlets at approximately 20 days post-hatch. This study was done within seagrass habitat in Aransas Bay, Texas during the settlement period (September to November).
The habitat is being analyzed to determine whether differences in structural or water quality characteristics influence patterns of larval abundance. The variation in fish abundance is likely a product of several factors,
including temporal variability in supply of larvae through the tidal inlet, differential survival and mortality, and movement within the seagrass bed.
Competition for space on growth, condition or predation of newly settled red drum larvae [Show Details]
Many marine fish species utilize seagrass meadows as nursery habitat during an early life stage, while other fishes complete their entire life cycle in this habitat. Compared to unvegetated areas, seagrass beds often
provide a more concentrated food supply, better protection from predators, and help prevent young fish from being advected to unfavorable habitat.
Because seagrass meadows provide habitat to many species of organisms, red drum larvae interact with the existing biota through foraging, predation, and competition. The role of competition in structuring populations of
seagrass-associated fishes is unclear, due to sampling difficulties. We modified traditional field-based approaches to meet the unique demands of working in a seagrass meadow. Our approach was to study interference
competition between cohorts of red drum in laboratory and field mesocosms. Young red drum are known to engage in aggressive behaviors such as chasing and biting, even when food resources are ample. In the field, large red drum
juveniles are frequently caught in the same area as small newly settled larvae.
This suggests that interference competition may affect the distribution of red drum in the field if competition leads to reduced growth rate, reduced condition, or increased predation risk.
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