Edward D. Houde

Ecosystem-Based Fishery Management

Ecosystem-based fishery management (EBFM) is a new direction for fishery management, essentially reversing the order of management priorities so that management starts with the ecosystem rather than a target species. EBFM aims to sustain healthy marine ecosystems and the fisheries they support. Pikitch et al . describe the potential benefits of implementation of EBFM that, in their view, far outweigh the difficulties of making the transition from a management system based on maximizing individual species.

The ctenophore Mnemiopsis in native and exotic habitats: U.S. estuaries versus the Black Sea basin

The native habitats of the ctenophore, Mnemiopsis, are temperate to subtropical estuaries along the Atlantic coast of North and South America, where it is found in an extremely wide range of environmental conditions (winter low and summer high temperatures of 2 and 32 °C, respectively, and salinities of <2–38). In the early 1980s, it was accidentally introduced to the Black Sea, where it flourished and expanded into the Azov, Marmara, Mediterranean and Caspian Seas. We compile data showing that Mnemiopsis has high potentials of growth, reproduction and feeding that enable this species to be a predominant zooplanktivore in a wide variety of habitats; review the population distributions and dynamics of Mnemiopsis in U.S. waters and in the Black Sea region; and examine the effects of temperature and salinity, zooplankton availability and predator abundance on Mnemiopsis population size in both regions, and the effects of Mnemiopsis on zooplankton, ichthyoplankton and fish populations, focusing on Chesapeake Bay and the Black Sea. In both regions, Mnemiopsis populations are restricted by low winter temperatures (<2 °C). In native habitats, predators of Mnemiopsis often limit their populations, and zooplanktivorous fish are abundant and may compete with the ctenophores for food. By contrast, in the Black Sea region, no obvious predators of Mnemiopsis were present during the decade following introduction when the ctenophore populations flourished. Additionally, zooplanktivorous fish populations had been severely reduced by over fishing prior to the ctenophore outbreak. Thus, small populations of potential predators and competitors for food enabled Mnemiopsis populations to swell in the new habitats. In Chesapeake Bay, Mnemiopsis consumes substantial proportions of zooplankton daily, but may only noticeably reduce zooplankton populations when predators of Mnemiopsis are uncommon. Mnemiopsis also is an important predator of fish eggs in both locations. In the Black Sea, reductions in zooplankton, ichthyoplankton and zooplanktivorous fish populations have been attributed to Mnemiopsis. We conclude that the enormous impact of Mnemiopsis on the Black Sea ecosystem occurred because of the shortage of predators and competitors in the late 1980s and early 1990s. The appearance of the ctenophore, Beroe ovata, may promote the recovery of the Black Sea ecosystem from the effects of the Mnemiopsis invasion.

Recent trends in estuarine fisheries: Predictions of fish production and yield

Trends in global and United States fish catches were examined to determine the status of estuarine fisheries yields relative to those from other ecosystems. Potential marine fish production, based upon primary production relationships, was estimated globally and for specific marine ecosystems, including estuaries. While global fish catches increased substantially during the past two decades and continued to increase through 1989, catches of estuarine-dependent species have peaked or stabilized. In the United States, total catches have increased but many estuarine-dependent fisheries have declined, although the declines in catches are no more dramatic than those of heavily-fished continental shelf species. Overfishing probably is the primary cause of declines in estuarine and shelf fisheries. A few estuarine-dependent species of the United States have experienced substantial increases in harvests since 1970, for example, Pacific salmons, menhaden, and penaeid shrimps. The percentage contribution of major estuarine fisheries to the United States commercial catch declined between 1970 and 1990, although the yield of these species increased substantially. Global marine fisheries production at trophic level 2.5 was estimated to be 1,359 million tons. Potential yield was estimated to be 307 million tons, but the 1989 world marine catch was only 86.5 million tons. The major fraction, 196 million tons, of the estimated potential yeild was for the open ocean where technological constraints may prevent its full realization. Of the remaining 111 million tons of the potential, 18.0 million tons (16.2%) may come from estuaries and probably already is fully exploited. The potential catches from shelves, 68.5 million tons (61.6%), and upwelling areas, 24.8 million tons (22.2%), while considerably larger than those from estuaries, are lower in a relative sense (per unit area) than fisheries production and potential catch in estuarine zones. Relationships between fish production, fish harvest, and primary production were examined in specific estuaries. The developing role of aquaculture and its effect on estuarine fisheries are discussed.

Feeding by marine fish larvae: developmental and functional responses

SynopsisThe relationship between prey consumption rate and prey concentration (functional response), and its change with growth (developmental response) were examined in the laboratory for three species of marine fish larvae: bay anchovy Anchoa mitchilli (Engraulidae), sea bream Archosargus rhomboidalis (Sparidae) and lined sole Achirus lineatus (Soleidae). The major objective was to determine relative predatory abilities of the larvae by fitting feeding rate data to developmental and functional response models. Feeding success, prey capture success, attack rates, handling times and search rates were estimated. Prey consumption rates and attack rates of bay anchovy usually were highest, but at the lowest prey level (50 per liter) first-feeding sea bream larvae had the highest consumption rate. Sea bream could consume prey at near-maximum rates at prey levels lower than those required by the other species. As larvae grew, time searching per attack decreased rapidly for all species, especially at low prey levels. Handling time also decreased, but most rapidly for bay anchovy. Search rates were highest for bay anchovy and lowest for lined sole. Bay anchovy had the best apparent predation ability, but when previous results on larval growth rates, survival rates and growth efficiencies were considered, sea bream larvae were the most efficient predators and the least likely of the three species to be limited by low prey levels.

Patterns and consequences of selective processes in teleost early life histories

Survival is variable and often unpredictable during the early lives of fishes. Consequently, numbers at transitional stages (e.g. metamorphosis) or at recruitment also are variable and difficult to forecast. In exploited stocks, recruitment variability frequently is the single most important factor affecting population fluctuations and the status of fisheries. For nearly a century, fishery scientists have sought to understand how numbers of young fish are regulated or controlled in complex aquatic ecosystems. Hypotheses, conceptual models, numerical models and experiments all have been directed at solving the ‘recruitment problem’. Progress is evident, but predicting recruitment remains a difficult-to-achieve and often elusive goal.

Temperature effects on the timing of striped bass egg production, larval viability, and recruitment potential in the Patuxent River (Chesapeake Bay)

The relationships between egg production (spawning behavior), larval growth and survival, and environmental conditions that larvae encounter were investigated in the Patuxent River tributary of Chesapeake Bay in 1991. Striped, bass (Morone saxatilis) eggs and larvae occurred predominantly above the salt front where conductivity was ≤800 μmhos cm−1. There were three prominent peaks in egg production, each coinciding with increasing temperatures. Estimated growth rates of 6-d, otolith-aged cohorts, which ranged from 0.15 mm d−1 to 0.22 mm d−1 (mean=0.17 mm d−1), were not demonstrated to differ significantly from each other. Observed zooplankton densities and temperature did not significantly affect growth rates. Stage-specific cumulative mortalities of combined cohorts were calculated for eggs (Zstage=0.20=18.1%), yolk-sac larvae (Zstage=5.80=99.7%), and first-feeding larvae (Zstage=2.95=94.8%). The very high mortality of yolk-sac larvae suggests that dynamic during this stage may have had a major impact on subsquent recruitment. Cohort-specific mortality rates of larvae were variable, ranging from Z=0.045 d−1 to 0.719 d−1, and were strongly temperature-dependent. Cohorts that experiented average temperature <15°C or >20°C during the first 25 d after hatching had significantly higher mortality rates than those which experienced intermediate temperatures. Estimated hatch-date frequencies of larvae ≥8 mm SL indicated goo, very good, and very low potential recruitments for cohorst spawned during early-season (April 2–11), mid-season (April 12–24) and late-season (April 25–May 5), respectively. Because seasonal temperature trends and fluctuations are unpredictable, striped bass females cannot select a spawning time that guarantees their offspring will be exposed to optimum temperatures. Consequently, selection may have occured for spawning over a broad range of temperatures and dates, a behavior insuring that some larval cohorts will encounter favorable temperatures.

Retention of white perch and striped bass larvae : Biological-physical interactions in Chesapeake Bay estuarine turbidity maximum

Physical and biological properties of the Chesapeake Bay estuarine turbidity maximum (ETM) region may influence retention and survival of anadromous white perch (Morone americana) and striped bass larvae (Morone saxatilis). To evaluate this hypothesis we collected data in five cruises, three during May 1998 and two during May 1999, in upper Chesapeake Bay. Time series of freshwater discharge, water temperature, wind, and water level explain differences in ETM location and properties between cruises and years. During high flows in 1998, a two-layer response to wind forcing shifted the ETM up-estuary, while a high discharge event resulted in a down-estuary shift in the salt front and ETM location. In 1999, extremely low discharge rates shifted the salt front 15 km up-estuary of its position in 1998. During 1999, the ETM was less intense and apparently topographically fixed. Gradients in depth-specific abundance of ichthyoplankton were compared with salinity and TSS concentrations along the channel axis of the upper Bay. During 1998, the high flow year, most striped bass eggs (75%) and most early-stage white perch larvae (80%) were located up-estuary of the salt front. In addition, most striped bass (91%) and white perch (67%) post-yolk-sac larvae were located within 10 km of maximum turbidity readings. Total abundance of white perch larvae was lower in 1999, a low freshwater flow year, than in 1998, a high flow year. In 1999, striped bass larvae were virtually absent. White perch (1977–1999) and striped bass (1968–1999) juvenile abundances were positively correlated with spring Susquehanna River discharge. The ETM regions is an important nursery area for white perch and striped bass larvae and life-history strategies of these species appear to insure transport to and within the ETM. We hypothesize that episodic wind and discharge events may modulate larval survival within years. Between years, differences in freshwater flow may influence striped bass and white perch survival and recruitment by controlling retention of egg and early-stage in the ETM region and by affecting the overlap of temperature/salinity zones preferred by later-stage larvae with elevated productivity in the ETM.

Nutrient enrichment and fisheries exploitation: interactive effects on estuarine living resources and their management

Both fisheries exploitation and increased nutrient loadings strongly affect fish and shellfish abundance and production in estuaries. These stressors do not act independently; instead, they jointly influence food webs, and each affects the sensitivity of species and ecosystems to the other. Nutrient enrichment and the habitat degradation it sometimes causes can affect sustainable yields of fisheries, and fisheries exploitation can affect the ability of estuarine systems to process nutrients. The total biomass of fisheries landings in estuaries and semi-enclosed seas tends to increase with nitrogen loadings in spite of hypoxia, but hypoxia and other negative effects of nutrient over-enrichment cause declines in individual species and in parts of systems most severely affected. More thoroughly integrated management of nutrients and fisheries will permit more effective management responses to systems affected by both stressors, including the application of fisheries regulations to rebuild stocks negatively affected by eutrophication. Reducing fishing mortality may lead to the recovery of depressed populations even when eutrophication contributes to population declines if actions are taken while the population retains sufficient reproductive potential. New advances in modeling, statistics, and technology promise to provide the information needed to improve the understanding and management of systems subject to both nutrient enrichment and fisheries exploitation.

Seasonality of occurrence, foods and food preferences of ichthyoplankton in Biscayne Bay, Florida

Ichthyoplankton and microzooplankton were collected twice monthly for one year at a single station in Biscayne Bay, Florida. Based on approximate 10-m3, 333-μm mesh bongo net samples, the mean annual densities were 17·7 m−3 and 1·8 m−3 for fish eggs and larvae, respectively. Ichthyoplankton was most abundant in spring-summer. The most common fish larvae were bay anchovy (Anchoa mitchilli), spotted dragonet (Callionymus pauciradiatus), thread herring (Opisthonema oglinum) and gobies (Gobiidae), which comprised 50% of all larvae collected. A comparison of 35-μm and 333-μm mesh, bongo net collections revealed that mean densities of fish larvae were 8·5 times higher in the smaller mesh. The most abundant microzooplankton, based on 35-μm bongo net collections, were copepod nauplii, <100 μm in width, which averaged 90·41−1 and tintinnids which averaged 168·51−1. The mean density of microzooplankton <100 μm wide, potentially suitable food for first-feeding fish larvae, was 104·91−1, exclusive of tintinnids, and 273·41−1 including tintinninds. Excepting tintinnids, seasonal variability in microzooplankton abundance was low relative to that for ichthyoplankton. High microzooplankton densities, combined with low seasonal variability, indicated that feeding conditions for fish larvae were usually good in Biscayne Bay. Copepods, especially nauplii, were the predominant food of fish larvae (71% of all food items). Only mollusc veliger larvae (18% of all food items) were highly preferred prey but they were a small component of most larval fish diets. Average width of prey eaten by first-feeding larvae was 74 μm. Prey size increased in relation to larval length.

Modeling particles and pelagic organisms in Chesapeake Bay: Convergent features control plankton distributions

A two-dimensional Lagrangian particle trajectory model is described and used to study how surface currents transport particles and, by analogy, plankton in Chesapeake Bay, United States. It is shown that persistent patches of high particle concentration develop in well-defined regions along the eastern shore and in the lower reaches of some western shore tributaries due to a combination of passive accumulation of particles in areas where the flushing rate is low and convergence. In the model the highest particle concentrations in the Chesapeake consistently develop in the lower bay (latitude 37.1°–37.7° N) in two specific regions near the shore of Cape Charles where convergence and downwelling occur. It is shown that one of these is associated with a strong and persistent, residual cyclonic eddy located over an abrupt change in bottom topography. Recent bay wide field surveys reveal that various planktonic groups, including phytoplankton, zooplankton, and bay anchovy eggs and larvae, have maximum abundances in the vicinity of this eddy. It is argued that these convergent areas are important features that have a strong influence on plankton distributions and that they provide consistently high food concentrations for higher trophic levels.