Population characteristics and potential factors influencing recruitment variability of blue catfish,channel catfish and flathead catfish in Thunderbird Reservoir,Oklahoma

Blue catfish (Ictalurus furcatus), channel catfish (Ictalurus punctatus) and flathead catfish (Pylodictis olivaris) are popular sport fishes throughout North America, and are therefore intensively managed by some natural resource agencies. Although catfish fisheries rely on sufficient natural reproduction and recruitment to sustain these populations, few studies have evaluated the variables affecting catfish recruitment. Thus, the objectives of this study were to describe population characteristics (age, size structure, condition, growth, mortality and recruitment) of blue catfish, channel catfish and flathead catfish in Thunderbird Reservoir, Oklahoma, and evaluate the effects of hydrology, water quality and temperature on year-class strength. During 2017 and 2018, 235 blue catfish, 194 channel catfish and 120 flathead catfish were collected and aged using otoliths. In general, these catfish species in Thunderbird Reservoir were slow growing, long-lived, had low mortality rates and experienced variable recruitment. Blue catfish recruitment was lower in years with higher average January air temperatures. Channel catfish recruitment was higher in years with increased average annual and spring/summer exchange rate and increased spring/summer average reservoir water volume (ha-m). Conversely, higher annual average total hardness (mg/L) resulted in decreased recruitment for channel catfish. Flathead catfish recruitment exhibited a minimal negative impact with increased annual reservoir volume (ha-m). The present study is among the first to describe potential variables influencing year-class formation of blue catfish, channel catfish and flathead catfish in reservoirs. Although the variables found to influence catfish recruitment are outside the control of fisheries managers, biologists should promote downstream angling opportunities for blue catfish that escape during reservoir water release events and stock channel catfish in systems with consistently high total water hardness in order to maximize fishing opportunities. Further, they can use the results of this study to manage angler expectations following periods of low recruitment.     

Carbon,nitrogen and phosphorus loadings from tilapia fish cages in Lake Malawi and factors influencing their magnitude

Lake Malawi is a great lake not only because of its large size (30,800 km²); but also because it hosts between 700 and 1000 fish species, mostly endemic cichlids. The fish are most abundant and more diverse in the littoral zone. Cage aquaculture in Lake Malawi started in 2004 in the nearshore waters of the southeast arm. Approximately 50 cages were operational by 2009 harvesting about 20 tons fresh fish/cage/year. In 2007, data were aggregated from cage feed and production records to produce a mass balance for carbon (C), nitrogen (N) and phosphorus (P) added to cages in the form of feed and juvenile fish for 22 production cycles. Nutrient losses from cages to the surrounding environment accounted for between 81 and 91% for C, 59 and 80% for N and 85 and 92% for P. Production strategies such as stocking density and feeding rates were also analysed to assess their effect on fish harvests and nutrient loads from the fish cages. The study showed that nutrient loads from the cages were significant and can be exacerbated by poor feed quality, overstocking, stocking of premature fish and use of lower than recommended feeding rates to grow the fish to target sizes. The inefficient production strategies necessitated longer production cycles (376 ± 42 days) and more feed usage to achieve market sizes of ≥ 300 g/fish than if generally recommended stocking and feeding rates were followed. If cage aquaculture continues to expand, it will become an important new source of nutrients, not only locally, but on a lake-wide basis.     

Fatty Acid Profiles Distinguish Channel Catfish from Three Reaches of the Lower Kaskaskia River and its Floodplain Lakes

Despite the increasing use of fatty acids (FAs) as biomarkers in aquatic food web analysis, little information is available regarding differences in FA profiles of fish among habitat types in river–floodplain ecosystems. The objectives of this study were to (i) test whether the FA profiles of channel catfish (Ictalurus punctatus) differed among three reaches of the lower Kaskaskia River and its floodplain lakes, and (ii) to compare FA profiles among muscle, liver, and adipose fin tissues collected from these fish. Profiles differed significantly among sites, especially between upper and lower river sites, and between river channel and oxbow lake sites, suggesting differences in FA availability for channel catfish occupying different habitats and river reaches in the Kaskaskia River system. Specifically, the essential FAs 18:2n‐6 and 18:3n‐3 increased in catfish tissues from upstream to downstream reaches, which could reflect increased floodplain connectivity and decreasing impoundment effects downstream. Ratios of n‐3 to n‐6 FAs were higher in fish from oxbow lakes, perhaps suggesting increased use of autochthonous production in the floodplain relative to the main river channel. Muscle and adipose fin FA profiles exhibited similar location‐related trends, whereas liver FA profiles were markedly different from the other tissue types. These results suggest that adipose fin tissue samples may be a viable, less‐invasive alternative to muscle tissue for analysis of FA profiles in channel catfish. Our study supports the use of tissue FA profiles in identifying habitat utilization by channel catfish, and perhaps habitat‐specific energy contributions to riverine consumers. Furthermore, our work highlights floodplain habitat as a potential source of essential n‐3 FA and the associated importance of maintaining river–floodplain connectivity to support aquatic food webs. Copyright © 2014 John Wiley & Sons, Ltd.     

Longitudinal patterns in flathead catfish relative abundance and length at age within a large river: effects of an urban gradient

We investigated the spatial variation of flathead catfish (Pylodictis olivaris) relative abundance and growth in the 274 km long Kansas River to determine if population dynamics of catfish are related to urbanization. Electrofishing was conducted at 462 random sites throughout the river in summer, 2005–2006 to collect fish. Relative abundance of age 1 fish (≤200 mm), subadult (>200–400 mm) and adult fish (>400 mm) ranged from 0.34 to 14.67 fish h^?1, mean length at age 1 was 165 (range: 128–195) mm total length (TL) and mean length at age 3 was 376 mm TL (range: 293–419 mm TL). The proportion of land use within 200 m of the river edge was between 0 and 0.54 urban. River reaches with high relative abundance of age 1 flathead catfish had high relative abundance of subadult and adult catfish. River reaches with fast flathead catfish growth to age 1 had fast growth to age 3. High urban land use and riprap in the riparian area were evident in river reaches near the heavily populated Kansas City and Topeka, Kansas, USA. Reaches with increased number of log jams and islands had decreased riparian agriculture. Areas of low urbanization had faster flathead catfish growth (r = 0.67, p = 0.005). Relative abundance of flathead catfish was higher in more agricultural areas (r = ?0.57, p = 0.02). Changes in land use in riverine environments may alter population dynamics of a fish species within a river. Spatial differences in population dynamics need to be considered when evaluating riverine fish populations. Published in 2008 by John Wiley & Sons, Ltd.     

Spatial structure of large‐river fish populations across main‐stem and tributary habitats

The spatial variability in demographic parameters represents fundamental information for conservation and management of large‐river fish populations. We assessed demographic processes including survival and movement across macroscale habitats in a large‐river network using 2 candidate large‐river species with contrasting life history strategies. We used mark–recapture data and a multistate model framework to estimate survival and transition probabilities between main‐stem and tributary habitats for both channel catfish and shovelnose sturgeon. Annual survival for channel catfish was similar in main‐stem and tributary habitats (range in S = 0.47–0.58). Annual survival for shovelnose sturgeon was less in the tributary (S = 0.68) compared with the main stem (S = 0.83). The probability of movement among macroscale habitats differed between species. However, the greatest probability of movement occurred from the tributary to the main‐stem for both channel catfish (ψ = 0.42) and shovelnose sturgeon (ψ = 0.27). Movement between main‐stem and tributary rivers may be a prominent characteristic for both channel catfish and shovelnose sturgeon and could influence population demographic rates and abundance across systems. Riverine fish populations are likely structured across multiple salient scales—including tributary and main‐stem habitats. Consideration of connectivity across tributary and main‐stem habitats with respect to species' life history strategy and life stage may better integrate a systems' perspective for conservation and management of large‐river fish populations.     

Recruitment Sources of Channel and Blue Catfishes Inhabiting the Middle Mississippi River

Insight into environments that contribute recruits to adult fish stocks in riverine systems is vital for effective population management and conservation. Catfishes are an important recreational species in the Mississippi River and are commercially harvested. However, contributions of main channel and tributary habitats to catfish recruitment in large rivers are unknown. Stable isotope and trace elemental signatures in otoliths are useful for determining environmental history of fishes in a variety of aquatic systems, including the Mississippi River. The objectives of this study were to identify the principal natal environments of channel catfish Ictalurus punctatus and blue catfish Ictalurus furcatus in the Middle Mississippi River (MMR) using otolith stable oxygen isotopic composition (δ¹⁸O) and strontium : calcium ratios (Sr : Ca). Catfishes were sampled during July–October 2013–2014, and lapilli otoliths were analysed for δ¹⁸O and Sr : Ca. Water samples from the MMR and tributaries were collected seasonally from 2006 to 2014 to characterize site‐specific signatures. Persistent differences in water δ¹⁸O and Sr : Ca among the MMR and tributaries (including the upper Mississippi, Illinois, and Missouri rivers as well as smaller tributaries) were evident, enabling identification of natal environment for individual fish. Blue and channel catfish stocks in the MMR were primarily recruited from the large rivers (Missouri and Mississippi) in our study area, with minimal contributions from smaller tributaries. Recruitment and year class strength investigations and efforts to enhance spawning and nursery habitats should be focused on in large rivers with less emphasis on smaller tributaries. Copyright © 2016 John Wiley & Sons, Ltd.     

生态清淤对于桥水库水质影响的数值模拟

以天津于桥水库为例,建立了考虑沉积物-水界面物质交换的二维水动力和营养物质输移模型,基于率定的参数模拟了不同水质背景条件下,水库库周清淤和全库清淤两种方案对水库中的氮、磷营养盐时空分布的影响。结果表明:模型能够可靠地模拟水库底泥氮、磷营养盐的动态释放及其与外界污染输入共同作用下水库的氮、磷营养盐的含量;底泥是于桥水库水体营养盐的重要来源,底泥清除后水库氮、磷营养盐含量均有所下降,但是不同方案的清淤效果存在差异。库周清淤对沿岸带的浅水区水体氮、磷有很好的改善作用,但对库区的整体改善效果不佳(<2%);而全库清淤后,整个库区水体的氮、磷含量均大幅下降(26%~48%),治理效果相对明显。     

Age‐0 Channel Catfish Ictalurus Punctatus Growth Related to Environmental Conditions in the Channelized Missouri River,Nebraska

Large river paradigms suggest that natural flow regimes are critical for maintaining instream habitats and promoting production and growth of native aquatic organisms. Modifications to the Missouri River, Nebraska, within the past 100 years have drastically reduced shallow water habitat, homogenized the flow regime, and contributed to declines in several native species. Despite drastic flow modifications, several metrics of the Missouri River's flow regime still vary across years. We related age‐0 channel catfish growth to environmental conditions in the channelized Missouri River, Nebraska, between 1996 and 2013 using an information theoretic approach. Growth rate was most influenced by growing season duration and duration of discharges below the 25th percentile of 30‐year daily Missouri River discharges. Periods of low water may be important for juvenile growth because of channel modifications that limit critical shallow water habitat during higher within‐bank flows. Exclusion of peak discharge and peak discharge timing in the best model to predict growth is counter to conventional thoughts on river fish responses to hydrological conditions but may be reflective of the general lack of high‐magnitude flooding during the majority of our study. Future efforts to relate juvenile fish growth to environmental conditions can provide guidance for water management in the Missouri River and other regulated North American rivers. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of damming a large lowland river on chironomids and fish assessed with the (multiplicative partitioning of) true/Hill biodiversity measure

The impact of a large lowland temperate zone reservoir on chironomid and fish biodiversity was investigated in the same upstream (U) and downstream (D) sites on the same sampling occasions in spring and autumn of several post‐impoundment years. The true/Hill N1 diversity measure was used to construct diversity models of the impact, and partitioning of N1 to reveal the importance of ecological gradients. N1 is unique in being decomposable among multiple levels and easily comparable across ecosystems and animal groups. Chironomids were more diverse than fish in all community sets owing to their probably lower mobility, lower interspecific competition, opportunistic character and much shorter life spans than those of fish. Fish exerted consumption pressure on chironomids upstream by foraging mostly on benthic insects, but not downstream where they fed on microcrustaceans of reservoir origin or on epiphytic fauna (mainly Chironomidae). The reservoir impact on chironomids decreased their diversity in U and increased in D, while much the opposite was true in fish. In D in macroinvertebrates, the impact consisted in advancing over time intensification of seasonal development of submersed macrophytes, while in D in fish in a few species escaping from the reservoir after their populational explosions each year. Partitioning of diversity revealed that the spatial (upstream–downstream) gradient was the strongest factor of diversity change in chironomids as compared with season, and age of the impoundment. No gradient was dominant in fish. Comparisons of observed data with a null model testified to very strong intraspecific aggregation in both chironomids and fish. Copyright © 2010 John Wiley & Sons, Ltd.     

Food‐web patterns and diversity in tropical fish communities

The food webs for three Sri Lankan reservoirs, Minneriya (ancient and shallow), Udawalawe (young and shallow) and Victoria (young and deep), were compared. The species richness of the fish communities was highest in Minneriya (30 species), intermediate in Udawalawe (21 species) and lowest in Victoria (18 species). The fish species belonged to 11 families, with Cyprinidae dominating the community in terms of both abundance and species richness. The daily quantity of food consumed per fish species was used to distinguish weak from strong trophic interactions in these food webs. The fish community consumption pattern was characterized by a few strong interactions and many weak ones. The number of major consumers (i.e. contributing >5% of the total fish community consumption) for each reservoir was small and similar for all three reservoirs. Dawkinsia singhala and Amblypharyngodon melettinus were the two major consumers in all three reservoir food webs. Puntius chola was a major consumer in Minneriya and Victoria, although not in Udawalawe, where it fed less on detritus than for the other two food webs. The fish community fed mainly at the bottom of the food web, primarily on algae, macrophytes and detritus in all three reservoirs, with very little piscivory occurring. The average food‐web length was measured as the mean trophic index weighted for the consumption rates of the various fish populations that together comprised the fish community. The average food‐web length in these three Sri Lankan reservoirs relatively short, compared with most other tropical lake/reservoir food webs in Asia and Africa for which relevant data were available. Furthermore, traditionally unexploited species (Oreochromis mossambicus; O. niloticus), in contrast to species of commercial fisheries interest, are major role players in the trophic dynamics of reservoir ecosystems. Thus, based on this study, the potential of their exploitation should be considered in the management of reservoir fisheries.     

Mercury Distribution in the Biota of a Great Lakes Estuary: Old Woman Creek,Ohio

A study of mercury contamination in a wetland ecosystem was undertaken to assess degree of contamination in various compartments of the food web, and determine the most important routes for mercury to enter fish species. Old Woman Creek is a freshwater estuary on the southern shore of Lake Erie and has been recognized as an important nursery area for fish. Although there is no point source of mercury to the wetland, there is evidence of biomagnification of mercury in the food web. Mercury concentrations in water were not detectable (< 20 ng/L) by methods used in this study. The sediments had a mean concentration of 0.04 /μg total Hg/g wet weight. Zooplankton, benthos, and macro-phytes had less than 0.05 μg total Hg/g wet weight, but mean concentrations in fish tissue ranged from 0.001 to 0.636 μg total Hg/g wet weight. There was a positive correlation between mercury concentration and both length and weight in channel catfish Ictalurus punctatus and common carp Cyprinus carpio. In general, channel catfish accumulated higher levels of mercury than common carp, suggesting that the pelagic food web may be more efficient in delivery of mercury (to channel catfish) than the benthic pathway (to common carp). The highest value for fish occurred in a bowfin Amia calva. Tissue from one great blue heron Ardea herodias had the highest concentration at 1.62 μg total Hg/g wet weight. In this system, as in others, top predators tended to accumulate the highest mercury levels and were therefore most susceptible to mercury poisoning, even at sublethal levels.     

Little Galloo Island,Lake Ontario: A Review of Nine Years of Double-crested Cormorant Diet and Fish Consumption Information

The diet of double-crested cormorants (Phalacrocorax auritus) on Little Galloo Island (LGI) in the eastern basin of Lake Ontario has been quantified since 1992. Over the past nine years considerable information has been generated on cormorant feeding ecology through the examination of approximately 12,000 pellets collected on LGI, where three distinct cormorant feeding periods, pre-chick, chick, and post-chick, are delineated by differences in diet composition and daily fish consumption. Yellow perch (Perca flavescens) were the major prey during pre-chick and post-chick feeding periods. Alewife (Alosa pseudoharengus), which move inshore to spawn in mid-June, dominated (>60%) cormorant diets during the chick feeding period. Mean daily fish consumption (14.6) during the pre-chick feeding period was significantly greater than during the chick feeding (9.3) or post-chick feeding (8.0) periods. The proportion of smallmouth bass (Micropterus dolomieu) in the diet increased over the season (0.8% to 7.2%), while the size of bass consumed declined (214 mm to 143 mm). Forage fish (mainly alewife, three-spine sticklebacks [Gasterosteus aculeatus] and minnows) comprised 58% of the diet of LGI cormorants, followed by panfish (37%) (yellow perch, pumpkinseed [Lepomis gibbosus], rock bass [Ambloplites rupestris]) and gamefish (5%) (mostly smallmouth bass). On the average LGI cormorants consumed about 32.8 million fish annually, weighing about 1.4 million kilograms. Cormorants from LGI consumed more biomass of smallmouth bass and yellow perch annually than is taken by sport (bass and yellow perch) and commercial (perch) fishermen.     

Seasonal Effects of Hydropeaking on Growth,Energetics and Movement of Juvenile Atlantic Salmon (Salmo Salar)

The performance and movements of juvenile Atlantic salmon Salmo salar exposed to variable water discharge (simulating hydropeaking) but with a stable water‐covered area were studied in six experimental stream channels, both during the winter and summer. Thirty fish were stocked into each channel, and the growth, body fat and movements of the fish were followed for about 2.5 months in each season. During the winter, no effect of hydropeaking was documented on performance or movement. In the summer, fish experiencing hydropeaking had lower body mass, lower body fat, and higher movement rates than the control fish. In general, effect sizes were small, and the rapid and frequent changes in water discharge and water level in the present study had small effects on the performance of juvenile Atlantic salmon. The cumulative long‐term effect at the population level is unknown, but a reduced growth rate of 10% and a reduction in body fat of 16% in the hydropeaking experiments in the summer might to some extent translate into increased smolt age and lower overwintering survival. Copyright © 2014 John Wiley & Sons, Ltd.     

水库运行对下游岸边带植被和鱼类的影响

本研究选取了漓江下游的一段复式河道为例,建立了一维全河流和二维局部河段的水环境模型,并与基于元胞自动机的岸边带植被模型和基于个体的鱼类模型耦合,从岸边带植被变化和鱼类种群动态角度研究水库运行对河流水生态系统的作用。研究以优势种植物水蓼、刺果酸模和益母草为对象,模拟了自然条件和水库调节作用下三种植物空间格局的演变,并选取草鱼和鲫鱼为研究对象,研究水库运行对鱼类生长和分布的影响。通过模拟结果的对比分析,发现水库运行有利于益母草的生长与扩张,而不利于水蓼的生长,水位波动对水蓼生长以及维持岸边植物多样性有正面作用。此外,假设仅考虑水库运行造成的下游水流变化的作用,枯水期水库向河道补水对草鱼有一定的正面影响,对鲫鱼有轻微的负面影响;而在在4-5月,水库运行对两种鱼类都存在比较明显的负面影响。本文建立的模型方法可为河流优化管理及水库生态友好运行提供支持。     

Energetic considerations for managing double-crested cormorants on Lake Champlain

We studied foraging distribution, activity budgets, fish consumption, and energetics of double-crested cormorants (Phalacrocorax auritus) at two breeding colonies on Lake Champlain. Our objective was to determine if fish consumption and distribution of predation changed with movements of cormorants associated with efforts to reduce numbers of cormorants on one of the colonies. Wildlife managers reduced populations of cormorants on Young Island, Vermont by oiling their eggs, which resulted in dispersal of breeding cormorants 35 km south to Four Brothers Islands, New York. We found that as cormorants shifted from Young Island to the colony on Four Brothers Islands, energy demands, foraging distribution, and total fish consumption increased. Birds on Four Brothers Islands foraged a greater distance from the colony compared to birds on Young Island. Additionally, consumption of yellow perch (Perca flavescens) shifted to rainbow smelt (Osmerus mordax) when more birds bred on Four Brothers Islands. The dispersal of cormorants from Young Island to Four Brothers Islands reduced predation on yellow perch but increased overall fish consumption. Our estimates of fish consumption ranged from 322,000–425,000 kg of fish per year at Young Island to 899,000–1,086,000 kg of fish per year at Four Brothers Islands. Results from this study demonstrate secondary impacts of management on Young Island to unmanaged areas.     

Refilling, ageing and water quality management of Brucher Reservoir

Refilling of the formerly oligo‐mesotrophic, softwater Brucher Reservoir commenced in April 1993 and took 11 months to completely fill. A severe ‘trophic upsurge’ in the sense of nutrient enrichment (phosphorus, dissolved organic material) as a result of the decomposition of the inundated vegetation occurred. However, algal crop and phosphorus utilization efficiency, expressed as chlorophyll concentrations per unit of total phosphorus, remained very low. In the absence of any fish stock, a single species, Daphnia galeata, monopolized the resources. Sustained by the detritus food chain, daphnids exerted a severe?‘top‐down’ control upon phytoplankton, thereby preventing any net algal growth. In 1994, artificial mixing prevented the occurrence of anoxic water conditions and internal nutrient enrichment of the lake. Although the decay of the inundated vegetation was still of importance, phosphorus concentrations in the water column approached equilibrium with the external input while dissolved organic material concentrations clearly declined. That year, the reservoir became stocked with minnows, sun bleak (Leucaspius delineatus) and trout. As food limitation, as a result of reduced heterotrophic production, became more severe in the face of an increasing predation pressure, the daphnid population density declined, resulting in a decreasing but still adequate community filtering rate providing pronounced ‘clear‐water phases’ of up to 10 m that were features of the period 1995–1997. Although D. galeata defended its key position in the food web, its life‐history traits (e.g. body size) changed. Submerged macrophytes, which since 1995 gradually colonized suitable areas of the reservoir, provided a favourable refuge for minnows from trout predation that resulted in reduced predation pressure upon pelagic daphnids. However, in 1998, ungrazeable algae became prominent, adversely affecting transparency. That year, the significant inverse relationship between chlorophyll : total phosphorus ratios and daphnids became uncoupled during the summer (July–August) by indigestible chlorococcalean algae. Nevertheless, the fishery management that was implemented was successful in sustaining not only the lowest yield of algae at the given nutrient concentration but also the most favourable species composition with respect to water quality.     

By-catch in the Saginaw Bay,Lake Huron commercial trap net fishery

This study provides species-specific catch and baseline mortality estimates of non-target species (by-catch) for the Saginaw Bay, Lake Huron commercial trap net fishery. By-catch can represent a significant mortality source that is often unknown. By-catch and by-catch mortality rates in the Saginaw Bay commercial trap net fishery, which primarily targets lake whitefish (Coregonus clupeaformis), yellow perch (Perca flavescens), and channel catfish (Ictalurus punctatus), are currently unknown. From May through August 2010, we observed onboard commercial trap net vessels and took species-specific counts of by-catch and estimated initial by-catch mortality (i.e., morbid or floating fish). The high levels of walleye (Sander vitreus) catch and mortality observed within inner Saginaw Bay have not been previously documented in the Laurentian Great Lakes. Walleye by-catch averaged 127.3 individuals per trap net lift and 42% of those caught were morbid. The levels of lake trout (Salvelinus namaycush) catch observed were within the range observed in previous studies, but mortality (percent) was higher than has been previously observed. Lake trout by-catch averaged 39.4 individuals per lift and 39.2% of those were morbid. Through the use of generalized linear models, this analysis also indicated factors that most influenced catch of non-target species including time of year and soak time (i.e., time interval between trap net lifts). Surface water temperature and trap net depth most influenced mortality. These results may inform fishers and fisheries managers and highlight the need for comprehensive by-catch monitoring throughout the Great Lakes.     

Relative demand by double-crested cormorants and anglers for fish production from lakes on Manitoulin Island,Lake Huron

The magnitude of angler harvest (kg·ha^− 1·yr^− 1) and cormorant consumption (kg·ha^− 1·yr^− 1) were compared for a set of lakes (N = 11) on Manitoulin Island. Empirical models relating total phosphorus to total fish production as well as production to body mass were used to scope the possible range of fish production and to partition production among small, medium and large size segments of fish populations, respectively. Medium (66–112 g) and large (> 200 g) size segments were defined as size categories targeted by cormorants (stomach diet analysis) and anglers (creel interviews), respectively. Angling effort and cormorant density were estimated from aerial surveys of the lake set during the open water season and for anglers during the winter ice-fishing season. Results showed that anglers harvested almost all large fish production, assuming the mean total fish production model, and 43% of large fish production under the more optimistic upper 80% prediction limit of total fish production. Cormorant consumption of medium fish production was less (39% using mean regression model; 15% using upper 80% prediction model) than angler consumption of large fish production. Anglers therefore imposed more population stress on their preferred sizes of fish than cormorants imposed on their preferred sizes of fish. Population stress was increased when cormorant consumption of medium size fish was discounted from contributing to large fish production. Angler harvest near (or above) sustainable yield levels will be exacerbated and appear as a fish collapse when cormorants consume fish production destined for fish size segments preferred by anglers.