Thomas J. Kwak

Food web analysis of southern California coastal wetlands using multiple stable isotopes

Abstract Carbon, nitrogen, and sulfur stable isotopes were used to characterize the food webs (i.e., sources of carbon and trophic status of consumers) in Tijuana Estuary and San Dieguito Lagoon. Producer groups were most clearly differentiated by carbon, then by sulfur, and least clearly by nitrogen isotope measurements. Consumer 15N isotopic enrichment suggested that there are four trophic levels in the Tijuana Estuary food web and three in San Dieguito Lagoon. A significant difference in multiple isotope ratio distributions of fishes between wetlands suggested that the food web of San Dieguito Lagoon is less complex than that of Tijuana Estuary. Associations among sources and consumers indicated that inputs from intertidal macroalgae, marsh microalgae, and Spartina foliosa provide the organic matter that supports invertebrates, fishes, and the light-footed clapper rail (Rallus longirostris levipes). These three producers occupy tidal channels, low salt marsh, and mid salt marsh habitats. The only consumer sampled that appears dependent upon primary productivity from high salt marsh habitat is the sora (Porzana carolina). Two- and three-source mixing models identified Spartina as the major organic matter source for fishes, and macroalgae for invertebrates and the light-footed clapper rail in Tijuana Estuary. In San Dieguito Lagoon, a system lacking Spartina, inputs of macroalgae and microalgae support fishes. Salicornia virginica, S. subterminalis, Monanthochloe littoralis, sewage- derived organic matter, and suspended particulate organic matter were deductively excluded as dominant, direct influences on the food web. The demonstration of a salt marsh–channel linkage in these systems affirms that these habitats should be managed as a single ecosystem and that the restoration of intertidal marshes for endangered birds and other biota is compatible with enhancement of coastal fish populations; heretofore, these have been considered to be competing objectives.

A Review of Tagging Methods for Estimating Fish Population Size and Components of Mortality

Abstract Techniques to improve estimation of animal population size and mortality from tagging studies have received substantial attention from terrestrial biologists and statisticians during the last 20 years. However, these techniques have received little notice from fisheries biologists, despite the widespread applicability to fisheries research, the wide variety of tag types used in fisheries research (from traditional fin clips to telemetry tags), and the development of new computer software to assist with analyses. We present a brief review of population models based on recaptures, returns, or telemetry relocations of tagged fish that can be used to estimate population size, total mortality, and components of mortality (i.e., fishing and natural) that are frequently of interest to fisheries biologists. Recommended strategies include (1) use closed population models (e.g., Lincoln-Peterson) to estimate population size for short term studies where closure assumption can be met, (2) use the robust desi...

Fish Assemblage Changes in an Ozark River after Impoundment: A Long-Term Perspective

Abstract We conducted an intensive fish survey in the tailwater reach of a large Ozark river 30 years after its impoundment and compared the recent fish assemblage with those prior to impoundment and shortly (4 years) after impoundment. Our primary objective was to assess whether relatively short-term monitoring following dam construction can adequately quantify the long-term effects of impoundment on downstream riverine fishes. The preimpoundment survey (1962–1963) described a fish assemblage composed of warmwater fish species, predominantly Cyprinidae, Ictaluridae, Centrarchidae, and Percidae. Yoke darter Etheostoma juliae (34%), central stoneroller Campostoma anomalum (24%), and Ozark madtom Noturus albater (7%) were the most abundant species. The postimpoundment surveys of 1965–1966 and 1968 documented immediate changes in the fish assemblage. No Ozark madtoms and only four yoke darters were collected shortly after impoundment. Central stonerollers accounted for 45–50% of the fish collected, and both ...

MODELING THE EFFECTS OF LAND USE AND CLIMATE CHANGE ON RIVERINE SMALLMOUTH BASS

Anthropogenic changes in temperature and stream flow, associated with watershed land use and climate change, are critical influences on the distribution and abundance of riverine fishes. To project the effects of changing land use and climate, we modeled a smallmouth bass (Micropterus dolomieu) population in a midwestern USA, large river–floodplain ecosystem under historical (1915–1925), present (1977–1990), and future (2060, influenced by climate change) temperature and flow regimes. The age-structured model included parameters for temperature and river discharge during critical seasonal periods, fish population dynamics, and fishing harvest. Model relationships were developed from empirical field data collected over a 13-yr period. Sensitivity analyses indicated that discharge during the spawning/rearing period had a greater effect on adult density and fishing yield than did spawning/rearing temperature or winter discharge. Simulations for 100 years projected a 139% greater mean fish density under a historical flow regime (64.9 fish/ha) than that estimated for the present (27.1 fish/ha) with a sustainable fishing harvest under both flow regimes. Simulations under future climate-change-induced temperature and flow regimes with present land use projected a 69% decrease in mean fish density (8.5 fish/ha) from present and an unstable population that went extinct during 56% of the simulations. However, when simulated under a future climate-altered temperature and flow regime with historical land use, the population increased by 66% (45.0 fish/ha) from present and sustained a harvest. Our findings suggest that land-use changes may be a greater detriment to riverine fishes than projected climate change and that the combined effects of both factors may lead to local species extinction. However, the negative effects of increased temperature and precipitation associated with future global warming could be mitigated by river channel, floodplain, and watershed restoration.

Trout Production Dynamics and Water Quality in Minnesota Streams

Abstract We sampled fish assemblages and quantified production dynamics of brook trout Salvelinus fontinalis, brown trout Salmo trutta, and rainbow trout Oncorhynchus mykiss in 13 southeastern Minnesota streams during 1988–1990 to examine the influence of water quality on fish populations in fertile trout streams. Fish assemblages in 15 stream reaches were abundant, but low in diversity; 13 species were collected. Parameter means (ranges) over the reaches were species richness, 4.1 (1–8); density, 29,490 (1,247–110,602) fish/ha; and biomass, 253.5 (49.6–568.6) kg/ha. Means (ranges) for salmonids were annual mean density, 2,279 (343–8,096) fish/ha; annual mean biomass, 162.0 (32.5–355.5) kg/ha; and annual production, 155.6 (36.7–279.6) kg/ha. Salmonid production and mean biomass were greater during the spring-fall interval than during fall-spring; young cohorts (ages 0–1) contributed the greatest proportion to population biomass and production. Salmonid annual production-to-mean-biomass ratio (P/B) average...

Largemouth bass mortality and related causal factors during live-release fishing tournaments on a large Minnesota Lake.

Abstract We quantified initial and delayed mortality of largemouth bass Micropterus salmoides during live-release fishing tournaments and identified probable causes of death in order to provide a biological basis for refining tournament guidelines and regulation. Mean estimates for two tournaments on Lake Minnetonka, Minnesota, were 1.42% weigh-in mortality, 3.35% 3-d delayed mortality, and 4.72% total mortality. Catch and total mortality rates during a May tournament were both over 1.5 times those of a September tournament, the difference presumably being related to reproductive behavior and condition. Tournament mortality of the population was minimal relative to other causes of mortality; total tournament mortality estimates for 1992 (11 tournaments) represented 2.3–6.3% of angling mortality and 1.3–3.0% of total mortality in Lake Minnetonka. Means of all water quality variables measured in live wells were significantly different from those of lake water, and relative differences were greatest for ammo...

Climate Change Effects on North American Inland Fish Populations and Assemblages

Climate is a critical driver of many fish populations, assemblages, and aquatic communities. However, direct observational studies of climate change impacts on North American inland fishes are rare. In this synthesis, we (1) summarize climate trends that may influence North American inland fish populations and assemblages, (2) compile 31 peer-reviewed studies of documented climate change effects on North American inland fish populations and assemblages, and (3) highlight four case studies representing a variety of observed responses ranging from warmwater systems in the southwestern and southeastern United States to coldwater systems along the Pacific Coast and Canadian Shield. We conclude by identifying key data gaps and research needs to inform adaptive, ecosystem-based approaches to managing North American inland fishes and fisheries in a changing climate.

Spatial Extent and Dynamics of Dam Impacts on Tropical Island Freshwater Fish Assemblages

Habitat connectivity is vital to the persistence of migratory fishes. Native tropical island stream fish assemblages composed of diadromous species require intact corridors between ocean and riverine habitats. High dams block fish migration, but low-head artificial barriers are more widespread and are rarely assessed for impacts. Among all 46 drainages in Puerto Rico, we identified and surveyed 335 artificial barriers that hinder fish migration to 74.5% of the upstream habitat. We also surveyed occupancy of native diadromous fishes (Anguillidae, Eleotridae, Gobiidae, and Mugilidae) in 118 river reaches. Occupancy models demonstrated that barriers 2 meters (m) high restricted nongoby fish migration and extirpated those fish upstream of 4-m barriers. Gobies are adapted to climbing and are restricted by 12-m barriers and extirpated upstream of 32-m barriers. Our findings quantitatively illustrate the extensive impact of low-head structures on island stream fauna and provide guidance for natural resource management, habitat restoration, and water development strategies.

Salinity Tolerance of Flathead Catfish: Implications for Dispersal of Introduced Populations

The flathead catfish Pylodictis olivarisis a large predatory fish that has been introduced widely beyond its indigenous range to the detriment of many native fish communities. It disperses rapidly within and among river systems, but its potential to use saltwater or brackish waters for migration and exploitation of marine prey resources is unknown. We evaluated the salinity tolerance of juvenile flathead catfish exposed to NaCl and synthetic seawater (0-34‰) in direct-transfer acute toxicity tests. The 96-h median lethal concentration (LC50) for fish exposed to NaCl at 188C was a salinity of 10.0‰ with a 95% confidence interval (CI) of 9.0-11.1‰, whereas the 96-h LC50 for fish in synthetic seawater (Instant Ocean) at 188C was 14.5‰ (95% CI, 13.7-15.5‰). To determine whether fish could survive the transition from freshwater to brackish water and then back to freshwater, fish were transferred from freshwater at 188C to water with a salinity of 8, 11, or 14‰ for 24 h and then returned to freshwater for 48 h. Fish exposed to synthetic seawater with a salinity of 8 or 11‰ for 24 h had more than 95% survival for 48 h after transfer to freshwater; however, no fish transferred directly to synthetic seawater with a salinity of 14‰ survived the 24- h exposure period. Additional acclimation tests with fish that were exposed to synthetic seawater in which salinity was increased daily by 4‰ yielded an LC50 of 15.8‰. Our results provide evidence that flathead catfish could tolerate exposure to many brackish waters along the Atlantic and Gulf of Mexico coasts of the United States and that the dispersal of introduced flathead catfish populations among rivers may not be limited by estuarine salinities. The flathead catfish Pylodictis olivaris is a large predatory catfish that has been introduced widely beyond its native range, including into coastal riv- ers of the Gulf of Mexico and the Atlantic slope of the United States. Native to the Mississippi, Mobile, and Rio Grande drainages and to portions of the Great Lakes region and northern Mexico,

Modeling Management Scenarios and the Effects of an Introduced Apex Predator on a Coastal Riverine Fish Community

Abstract The flathead catfish Pylodictis olivaris, a carnivorous fish species native to most of the central interior basin of North America, has been introduced into at least 13 U.S. states and 1 Canadian province. Concurrent declines in abundance of native fishes have been reported in aquatic systems where flathead catfish have been introduced. To evaluate the potential impact of this invasive species on the native fish community we developed an ecosystem simulation model (including flathead catfish) based on empirical data collected from a North Carolina coastal river. The model results suggest that flathead catfish suppress native fish community biomass by 5–50% through both predatory and competitive interactions. However, our model suggests these reductions could be mitigated through sustained exploitation of flathead catfish by recreational or commercial fishers at rates equivalent to those for native flathead catfish populations (annual exploitation = 6–25%). These findings demonstrate the potential...