Spotted Seatrout Spawning Requirements and Essential Fish Habitat: A Microhabitat Approach Using Hydrophones.

Donald M. Baltz. Oceanography and Coastal Sciences,
Coastal Fisheries Institute, Louisiana State University, Baton Rouge, LA 70803, USA.

Hyrdrophones can be used in conjunction with a microhabitat approach to yield a fish’s eye view of its habitat requirements. At the finest scale, the microhabitat of an individual is the site it occupies at a given point in time. Sites are presumably selected to optimize an individual’s net energy gain while avoiding predators (i.e., tradeoffs of growth vs mortality). Since similarly sized individuals of a species select similar microhabitats, many careful measurements of individuals and associated physical, chemical, and biological variables should define the population's responses to environmental gradients. As defined here, the microhabitat is an occupied site, not a little bitty habitat type. Fine-scale measurements of environmental conditions at a site occupied by one or more individuals constitute an observation, and many independent observations characterize the population’s response to complex gradients.

Habitat is a loosely used ecological term that can be applied at the individual, population, and community levels and is often entangled with so many other ecological concepts that it can mean everything and therefore nothing. The term ‘habitat’ has almost been relegated to the status of a pseudocognate (sensu Salt 1979, Ecology) in that it is a term in common use and each individual who uses it feels that all others share his own intuitive definition. Nevertheless, it is used and can be useful. We can use habitat as the range of environmental conditions in which a species/population/life-history stage can live. It is a general term that broadly defines where a species lives without specifying patterns of resource use (sensu Hurlbert 1981, realized niche = resources used: energy, materials, and sites (Evolutionary Theory 5:177-184)). There are several points of view. From a fish's point of view, its distribution over environmental gradients describes its habitat. From a biologist's point of view, strata in the environment can be arbitrarily described as habitats, but more properly as "habitat types". At the community level, the environments dominated by a single species (e.g., Spartina alterniflora) may be characterized as Spartina habitat, but more properly as a "Spartina community".

The concept of Essential Fish Habitat (EFH) is based on 1996 federal legislation and is aimed at enhancing the sustainability of our fisheries. The legislation established four levels of data quality in defining EFH: I. Presence/absence, II. Density patterns (e.g., population responses to gradients, suitability), III. Condition/health (e.g., growth, parasite loads, pollution loads, RNA/DNA ratios), and IV. Production (e.g., secondary production, reproductive output). What is EFH? Essential is a qualifier that carries a notion of quality. We are not just asking where a species lives (Level I), but where it lives well (Levels II-IV). Where are its resource needs best met? Now we are concerned with patterns of resource use (sensu Hurlbert 1981, realized niche = resources used: energy, materials, and sites (Evolutionary Theory 5:177-184)).

How can we use a fish’s eye view to define EFH? We can seek answers to three questions. How are species and life history stages distributed in the environment? What intervals along environmental gradients are selected or avoided? What is most important to the fish in terms of growth and/or survival?

Level I data: Presence/Absence data. Most risk averse approach to protecting habitat (based on the precautionary principle); however, reliance on Level I data overprotects less valuable habitat and essentially equates water with EFH. High quality habitat is given the same level of protection as Low quality habitat and scientists/managers lose credibility.

Level II data: Density data. Uses fish population’s responses (density patterns) to environmental gradients. Level II assessments can be improved by relating population’s response in terms of resource use to resource availability (e.g., habitat suitability), and high quality can be distinguished from low quality habitat. Habitat Suitability [S = Suitability = P (E | F) / P (E)] is an index of habitat quality based on a quotient of Resource Use and Resource Availability (Bovee, K. D. & T. Cochnauer, 1977, U.S. Fish & Wildlife Service Biological Services Program FWS/OBS-77/63). Resource use is a probability statement, given the presence of fish, and resource availability is a probability statement, regardless of the presence of fish. Suitability is an index of use divided by availability that ranges from zero (intolerable) to one (optimal) after standardization.

Level III data: Growth data. I have not been able to relate this level to hydrophone work on spotted seatrout, but others may find an application for other soniferous species that make sounds for non-reproductive functions (e.g., foraging parrotfishes). What environmental conditions foster growth of early juvenile spotted seatrout? Nursery microhabitat selection is presumably controlled by some combination of physiological constraints, prey distributions, foraging success, competitor densities, and predation pressure, all of which may influence growth and/or survival. Linkages between microhabitat, diet, & conspecific density may predict recent daily growth which in turn reveals the recruitment potential of preferred nursery characteristics (Baltz et al.1998, Env Biol Fish 53: 89-103).

Level IV: Production data. This is the best kind of information and the best example if from a study of oyster seed production (Chatry, Dugas, and Easley 1983, Cont. Mar. Sci. 26: 81-94). Oyster seed set and growth are best at 20-22 ppt (Level III data), but oyster predators (drills, etc) seriously deplete populations in high salinity water (> 15 ppt). Oyster seed production is highest for seed set at 12-16 ppt the previous summer, and therefore EFH for oyster seed production is highest in a narrow summer salinity range of 12-16 ppt.

I will argue that suitability indices for reproduction (Saucier & Baltz, 1993, Env Biol Fish 36: 257-272) are high-level EFH data and qualify at Level IV. Moreover, this kind of data can be acquired easily with hydrophones. Saucier and Baltz (1993) used a microhabitat approach to identify selected and avoided points along salinity, depth, substrate, and velocity gradients used for spawning by spotted seatrout. Relatively deep, moving waters with a salinity of 14-23 and a temperature of 29-33 °C were selected. Conflicting literature from earlier studies in the northern Gulf of Mexico suggested that spawning occurred more or less exclusively in bays, passes or the open gulf. We found that spotted seatrout spawn across a wide variety of habitat types (bays, channels, passes, and open gulf) where environmental conditions are right. We found that spawning locations shifted along a salinity gradient up to 30 km on a north-south axis, and concluded that environmental conditions were more important than places.

Spawning temperatures for Louisiana spotted seatrout

Spawning salinity for Louisiana spotted seatrout

There are several pitfalls to avoid. Non-linear effects along environmental gradients should be expected. Non-representativeness in sampling design may lead to biased results, especially sampling bias that focuses on particular habitat types may generate misinformation. Noisy crews, boats and traffic may make it difficult to locate spawning aggregations. Misidentification of drumming species can be avoided by careful comparisons with known recordings, and verification of actual spawning by the collection and rearing of eggs from drummng sites to identifiable larvae is important. A stratified water column may present contrasting environmental variables in a vertical profile (We want to know what’s going on at fish’s nose).

Hydrophone Techniques & Assumptions. Aggregation size: Sound Intensity may be used to estimate Source Level (SL) if distance to source is known. Source Level is calculated by adding a one-way spherical spreading loss (i.e., a 20 log [depth in meters - 1]) for a correction (absorption is ignored). It is a continuous variable that estimates group size for statistical modeling: SL = microhabitat variables + temporal variables + e. A recorded Sound Intensity (my standard settings were 132 db re 1 m pascal) of +5 db yields a Source Level of 139.6 db for an aggregation on the bottom in 15 m of water [e.g., 132 + 5 + (20 log 14) = 139.6 db]. A cylindrical correction or no correction may be more appropriate under given circumstances.

Future Applications. My wish list is topped by a fixed or moveable listening array with overlapping directional capabilities to generate position fixes, computer programs to process fix data, and real-time transmission capabilities to allow a small boat to move to aggregation sites and random sites for measurements of resource use and availability.

Recommendations for EFH. Quality research and wise management related to fish habitat depend on how clearly we can define habitat and EFH and that we are all discussing the same concepts. We should try to take a fish’s point of view and let them describe what is essential along environmental gradients. By comparing resource use with environmental availability, we gain insights into patterns of selection and avoidance. We can identify biological endpoints that reflect the health and well being of individuals and communities of fishes. Use of the best data and research designs available will help avoid management errors in describing EFH. Management errors that result in over- or under-protecting EFH can be viewed as having positive and negative outcomes:

Clearly, we can use hydrophones and a microhabitat approach to credibly describe EFH for some soniferous spawning fishes, like spotted seatrout.

Acknowledgments. I am grateful to Grant Gilmore and Mike Mok for an acoustical primer and other assistance, Scott Holt for showing us how to rear larvae, Louisiana Sea Grant for funding, and Rodney Rountree, Tony Hawkins, & Cliff Goudey for putting together the workshop.

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