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1.
The double‐crested cormorant, Phalacrocorax auritus, is considered the primary depredating bird species on commercially produced channel catfish, Ictalurus punctatus, in the southeastern USA. We simulated different levels of cormorant predation on losses at harvest and economic effects on channel catfish production in a multiple‐batch cropping system. We observed significant (P < 0.05) declines in catfish production at increasing levels of cormorant predation in this study. This decline was mitigated by increased individual growth of catfish at higher predation rates (i.e., lower catfish densities). This mitigating effect produced a non‐linear relationship with total kg of catfish harvested per pond resulting in a non‐linear incremental increase in breakeven price related to predation. Costs of production ($/kg) increased with increasing predation levels up to very high levels of predation with a cumulative maximum increase in breakeven price of $0.143/kg. These results indicate that losses at harvest due to cormorant predation occur immediately but are mitigated in part by compensatory growth of individual catfish. Losses due to cormorant predation in multi‐batch systems can be considerable, but there is not a 1:1 relationship between losses and kg of catfish harvested due to compensatory factors.  相似文献   

2.
Double-crested cormorant Phalacrocorox auritus Lesson (cormorant) populations have increased throughout the Great Lakes region of North America causing concern related to the impact of cormorant predation on fish communities. A recent decline in yellow perch Perca flavescens (Mitchill) abundance within the Lake Winnebago System, Wisconsin, USA, prompted an assessment of cormorant diets to evaluate potential effects of cormorant predation on the sportfish community. Diets were collected from 883 cormorants (417 from Lake Winnebago and 466 from Lake Butte des Morts) between 2015 and 2017. Cormorant diets on both waterbodies consisted mostly of freshwater drum Aplodinotus grunniens Rafinesque and gizzard shad Dorosoma cepedianum (Lesueur). Yellow perch and walleye Sander vitreus (Mitchill) observations were infrequent and represented < 5% of cormorant diets by weight each year. Under current conditions, cormorant predation likely has minimal impact on the Lake Winnebago sportfish community, but more research is needed to assess potential impacts on Lake Butte des Morts.  相似文献   

3.
Climate change has been identified as a global threat to Arctic char Salvelinus alpinus L. populations. Bayesian statistical models were used to identify important physical and ecological factors explaining Arctic char presence and persistence in Irish lakes. Maximum lake depth and the presence of mixed fish communities (i.e. including non‐salmonid fish species) were the most important drivers of Arctic char presence. There was a 75% probability that an Irish lake would contain Arctic char when maximum depth exceeded 40 m, if only a salmonid community was present. However, the required depth increased to 57 m when a mixed fish community (e.g. pike, perch or roach) was present. Similar variables explained char persistence, with surviving Arctic char populations being more likely in lakes with greater maximum depth and fewer non‐salmonid fish species. Tested temperature covariates were not important in explaining Arctic char presence or persistence in Irish lakes. A risk analysis was undertaken to identify which Arctic char lakes are vulnerable to colonisation by thermally plastic mixed fish communities due to flooding. Results indicated that 32 out of 45 lakes were not at any risk from the colonisation of thermally plastic mixed fish communities and 13 lakes had some level of risk.  相似文献   

4.
The Atlantic salmon (Salmo salar) population of the River Minho represents the southern natural distribution edge of the species. In line with the general trend for Atlantic salmon, this population has been declining over the years and is now at a critically low level. With river connectivity compromised by a large dam just 80 km upstream the River Minho's outlet, and an expected deterioration of climatic conditions, it is urgent to increase our knowledge of this population and identify survival bottlenecks that can be addressed. In this study, we used radio and acoustic telemetry to track Atlantic salmon smolts during their migration towards the sea and record both survival rates and possible causes of mortality. The recorded survival for the tagged migrating Atlantic salmon remained below 55% in the three studied years, indicating that the in‐river loss of smolts is likely a strong constraint to this population. From the smolts to which a likely cause of mortality could be attributed (34%), most appear to have been removed from the river (25%), with two confirmed events of bird predation and one of mammal predation. Interestingly, eight tags were recorded moving back upstream, likely indicating predation by larger fish. Increasing predator populations (e.g. cormorants, Phalacrocorax carbo) and invasive predators (e.g. American mink, Neovison vison) lead to elevated predation pressure on this already strained Atlantic salmon population, and further studies quantifying their impact in more detail could prove crucial for future management considerations.  相似文献   

5.
Results from an acoustic telemetry study revealed for the first time a northerly migration route for Atlantic salmon (Salmo salar L.) smolts leaving the east coast of Ireland. Atlantic salmon smolts were tagged in spring 2019 in the Castletown and Boyne rivers. Three tagged smolts registered on disparate deep‐water offshore marine receivers as they travelled northwards out of the Irish Sea through the North Channel. One fish had migrated an estimated 250 km in a period of 32 days. The remaining two individuals were detected on receivers located off the Northern Ireland coast, further corroborating the northward migration of salmon smolts through the Irish Sea.  相似文献   

6.
Substantial economic losses of farmed catfish to fish‐eating birds such as the double‐crested cormorant, Phalacrocorax auritus, continue to be reported on U.S. catfish farms. An economic analysis was conducted of the on‐farm effects of both the increased expenditures to scare fish‐eating birds from catfish farms and of the value of the catfish that were consumed by cormorants. A survey was conducted of U.S. catfish farmers in the Delta region of Mississippi and Arkansas, to obtain farm‐level data on expenditures to scare birds. Estimations of the lost revenue from catfish consumed by cormorants were developed from a concurrent study on cormorant distribution, abundance, and diet in the region. The economic effects of bird predation in terms of both fish consumption and management costs were evaluated across three farm sizes and nine catfish production practices. Catfish farmers spent on average $704/ha ± $394/ha to scare birds, making bird‐scaring costs one of the top five costs of raising catfish. The greatest cost components of scaring birds were manpower (39% of all bird‐scaring costs) and the variable and fixed costs of trucks used to scare birds (34% of all bird‐scaring costs). Losses were greater on hybrid than channel catfish fingerling ponds. Industry‐wide, the value of catfish losses averaged $47.2 million (range of $25.8–$65.4 million). Total direct economic effects (including both the increased costs to scare birds and the revenue lost from fish consumed by cormorants despite bird‐scaring attempts) averaged $64.7 million (ranging from $33.5 to $92.6 million). Profitability improved by 4% to 23% across the farm size/production strategies analyzed upon removal of the economic effects from bird predation, with greater effects occurring on smaller‐scale farms. One‐third of the farm size and production scenarios analyzed changed from being unprofitable to showing a profit in the absence of such negative economic effects associated with bird depredation. Overall, the combined effects of increased farm expenditures to scare birds from farms and the value of the catfish lost to predation by cormorants caused substantial negative economic effects on catfish farms.  相似文献   

7.
  • 1. The question as to whether no‐take zones, as reference areas in the Wadden Sea (eastern North Sea), may develop within a few years or decades towards an ecological situation similar to the one which may have existed in earlier times is examined.
  • 2. It is argued that the driving forces behind changes in species and habitats in the Wadden Sea are natural long‐term variability of abiotic factors and area‐wide anthropogenic impact (coastal engineering). 3. Fisheries in the Wadden Sea are shown to be of minor importance, and it is proposed that establishment of no‐take zones here will not result in the recovery and recolonization of missing species and habitats. The examples of species include European oysters (Ostrea edulis), sabellarian reefs (Sabellaria spinulosa) and white weed (Sertularia cupressina, Hydrallmania falcata).
Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

8.
The Wadden Sea is an extensive wetland area, recognized as UNESCO world heritage site of international importance. Since the mid‐1990s, the invasive Pacific oyster Crassostrea gigas (Thunberg 1793) population in the area has grown exponentially, having a distinct impact on the ecosystem. The recent spread of the emerging oyster pathogen Ostreid herpesvirus OsHV‐1 μVar worldwide and specifically in the oyster culture areas in the south of the Netherlands raised the question whether the virus may also be present in the Wadden Sea. In the summer of 2012 juvenile Pacific oysters were collected from five locations in the Dutch Wadden Sea. The virus was shown to be present in three of the five locations by real‐time PCR and sequencing. It was concluded that OsHV‐1 μVar has settled itself in Pacific oyster reefs in the Wadden Sea. These results and the recent discoveries of OsHV‐1 microvariants in Australia and Korea indicate that OsHV‐1 μVar and related variants might be more widespread than can be deduced from current literature. In particular in regions with no commercial oyster culture, similar to the Wadden Sea, the virus may go undetected as wild beds with mixed age classes hamper the detection of mortality among juvenile oysters.  相似文献   

9.
Abstract Observations relevant to the North American stock complex of Atlantic salmon, Salmo salar L., suggest that marine mortality is influenced by variation in predation pressure affecting post‐smolts during the first months at sea. This hypothesis was tested for Gulf of Maine (GOM) stocks by examining wind pseudostress and the distribution of piscivorous predator fields potentially affecting post‐smolts. Marine survival has declined over recent decades with a change in the direction of spring winds, which is likely extending the migration of post‐smolts by favouring routes using the western GOM. In addition to changes in spring wind patterns, higher spring sea surface temperatures have been associated with shifting distributions of a range of fish species. The abundance of several pelagic piscivores, which based on their feeding habits may predate on salmon post‐smolts, has increased in the areas that serve as migration corridors for post‐smolts. In particular, populations of silver hake, Merluccius bilinearis (Mitchell), red hake, Urophycis chuss (Walbaum), and spiny dogfish, Squalus acanthias L., increased in size in the portion of the GOM used by post‐smolts. Climate variation and shifting predator distributions in the GOM are consistent with the predator hypothesis of recruitment control suggested for the stock complex.  相似文献   

10.
Some fish parasites constitute severe management problems as they may cause mortality of their fish host or are important zoonoses of humans. Parasite assessments are therefore critical to keep track of infections. If conventional sampling techniques can be simplified, parasite assessments might be easier to obtain, less time‐consuming and more extensive. In this study, we compare the assessed number of Diphyllobothrium spp. cysts (CYST) with the counted number of Diphyllobothrium spp. plerocercoid larvae recovered using a conventional digestive technique (LARV). The aim was to determine the potential of using CYST as a simplified methodology for assessing Diphyllobothrium spp. infection in salmonids. In total, 365 brown trout and 424 Arctic charr were sampled from nine lakes in subarctic Norway. Strong correlation, significant linear relationship and large amount of explained variation were found between log10CYST and log10LARV in both fish species. The method had a slight, but not significant tendency to work better in charr compared to trout. In addition, absolute difference between CYST and LARV increased at parasite intensities >100 indicating that the method has reduced functionality when estimating parasite intensity in heavily infected salmonid populations. However, overall, using this simplified and less time‐consuming methodology, a good indication of Diphyllobothrium spp. intensity, abundance and prevalence was obtained. We suggest that this method provides a sound proxy of the Diphyllobothrium spp. burden and have the potential to be used in parasite assessment during fish monitoring and fisheries management surveys, particularly if the time and resources for detailed parasite studies are not available.  相似文献   

11.
Temperature and body size are widely agreed to be the primary factors influencing vital rates (e.g., growth, mortality) in marine fishes. We created a biophysical individual‐based model which included the effects of body size and temperature on development, growth and mortality rates of eggs, larvae and juveniles of Atlantic cod (Gadus morhua L.) in the North Sea. Temperature‐dependent mortality rates in our model were based on the consumption rate of predators of cod early‐life stages. The model predicted 35%, 53% and 12% of the total mortality to occur during the egg, larval and juvenile stages, respectively. A comparison of modeled and observed body size suggested that the growth of survivors through their first year of life is high and close to the growth rates in ad libitum feeding laboratory experiments. Furthermore, our model indicates that experiencing warmer temperatures during early life only benefits young cod (or theoretically any organism) if a high ratio exists between the temperature coefficients for the rate of growth and the rate of mortality. During the egg stage of cod, any benefit of developing more rapidly at warmer temperatures is largely counteracted by temperature‐dependent increases in predation pressure. In contrast, juvenile (age‐0) cod experiences a higher cumulative mortality at warmer temperatures in the North Sea. Thus, our study adds a new aspect to the ‘growth–survival’ hypothesis: faster growth is not always profitable for early‐life stages particularly if it is caused by warmer temperatures.  相似文献   

12.
Herring (Clupea harengus) enter and remain within North Sea estuaries during well‐defined periods of their early life history. The costs and benefits of the migrations between offshore spawning grounds and upper, low‐salinity zones of estuarine nurseries are identified using a dynamic state‐variable model, in which the fitness of an individual is maximized by selecting the most profitable habitat. Spatio‐temporal gradients in temperature, turbidity, food availability and predation risk simulate the environment. We modeled predation as a function of temperature, the optical properties of the ambient water, the time allocation of feeding and the abundance of whiting (Merlangius merlangus). Growth and metabolic costs were assessed using a bioenergetic model. Model runs using real input data for the Scheldt estuary (Belgium, The Netherlands) and the southern North Sea show that estuarine residence results in fitter individuals through a considerable increase in survival probability of age‐0 fish. Young herring pay for their migration into safer estuarine water by foregoing growth opportunities at sea. We suggest that temperature and, in particular, the time lag between estuarine and seawater temperatures, acts as a basic cue for herring to navigate in the heterogeneous space between the offshore spawning grounds at sea and the oligohaline nursery zone in estuaries.  相似文献   

13.
Nasmith LE, Tonn WM, Paszkowski CA, Scrimgeour GJ. Effects of stocked trout on native fish communities in boreal foothills lakes.
Ecology of Freshwater Fish 2010: 19: 279–289. © 2010 John Wiley & Sons A/S Abstract – Ecological effects of stocking nonnative trout into lakes are receiving increased attention, especially in alpine environments. We assessed effects of stocked trout on native forage fishes in the boreal foothills of Alberta (Canada) by comparing fish density, population size structure and spatial and temporal activities in stocked and unstocked lakes over 3 years (2005–2007). The numerically dominant dace (primarily Phoxinus spp.) were larger in stocked lakes, consistent with size‐limited predation. Dace were also more crepuscular and concentrated on the lake‐bottom in stocked lakes, compared to more daytime activity in the water column in unstocked lakes. There were, however, no demonstrable effects of trout on the abundance of forage fish. The lack of major population‐level impacts of stocked trout suggests that current stocking practices, characteristics of boreal foothill lakes (e.g. thermal structure, abundant invertebrates, dense macrophytes) and/or behavioural adjustments of forage fish contribute to healthy native fish populations in our stocked lakes.  相似文献   

14.
Artificial structures can protect fish against predation by cormorants (Phalacocorax spp.). However, their effectiveness in larger water bodies with different fish communities in the presence of natural vegetation still needs to be explored. Using a large‐scale field experiment with 24 ponds stocked with differently composed fish communities, the present study investigates the extent to which the effect of artificial refuges on fish is species‐specific and determined by the characteristics of the fish community. This study provides strong experimental evidence for artificial refuges protecting fish against predation from cormorants, even in the presence of submerged vegetation. The effect of refuges was, however, highly species‐specific and depended on the composition of the fish community. Strong positive effects of refuges on rudd, Scardinius erythrophthalmus (L.), and roach, Rutilus rutilus (L.), populations were observed, especially in ponds where these species dominated. Overall, the total biomass of young‐of‐the‐year, 1‐year‐old and adult rudd and roach was on average 500, 7 and 15 times lower in ponds without than in ponds with refuges, respectively. No effect of artificial refuges on other fish species was found. This study indicates that artificial refuges can facilitate the coexistence of predation vulnerable fish populations with cormorants in lakes and ponds.  相似文献   

15.
In this study, 221 two‐year‐old hatchery‐reared salmon, Salmo salar, smolts were tagged with radio transmitters over a period of three consecutive years and released in the river in groups of 20–21 fish in various dates between late April and early June. Tagged smolts were tracked during their downstream migration in the lower 36‐km stretch of the regulated River Oulujoki, with the focus on the effects of release date, water temperature and river flow on migration behaviour and survival. The results indicate that release timing and river temperature have profound effects on the initiation of migration, swimming speed and survival of released S. salar smolts. Smolts released early in the spring in cold waters ceased migration after brief downstream movement and were vulnerable to predation, whereas the migration speed and survival rates increased markedly for smolts released later in the spring.  相似文献   

16.
Life cycle closure for species inhabiting areas with daily varying currents but directed net water transport requires specific behavior to minimize losses due to advection of passive drifting life stages. Variations in swimming activity of different‐sized Crangon crangon (15–65 mm total length) were therefore monitored under constant laboratory conditions immediately after being caught in the German Wadden Sea. Activity of shrimps of different sizes, caught at different seasons, always peaked at times corresponding with ebb tide in the habitat from where they were taken. This behavior was maintained for several days if no external stimuli were present but shifted to night activity if a light–dark cycle was provided. The observed behavior/activity pattern was included in a coupled hydrodynamic and individual‐based model (IBM) and the shift in the location of a shrimp cohort was monitored over time. Performance of ebb tide activity not only allowed the shrimps to reach the preferred deeper winter and spawning areas but also allowed them to migrate against the dominating current from eastern nurseries to more western located spawning areas. Passively drifting larvae released at these locations and later larval and juvenile stages that perform flood tide transport can reach the nurseries again. This links the nurseries and adult spawning grounds and closes the migration triangle.  相似文献   

17.
  1. Conservation of riverine fish often aims to improve access to spawning grounds and restore longitudinal connectivity by removing migration barriers, and involves substantial investments. However, these investments also enable non‐native predators to invade upstream into spawning areas and potentially adversely affect the recruitment of threatened freshwater fish through egg or fry predation.
  2. Detecting egg predation is often challenging. Visual inspections of fish gut contents may underestimate predation of soft materials such as eggs and fry, which limits the discovery of predators preying upon these life‐stages. DNA‐based detection assays may offer a more sensitive tool to assess predation of soft materials.
  3. A conservation issue was confirmed by developing and applying a species‐specific DNA‐based detection assay: invasive round goby (Neogobius melanostomus) prey on the eggs or fry of the threatened common nase (Chondrostoma nasus) in Switzerland.
  4. DNA‐based detection assays were also developed for five other valuable native fish species, including endangered salmonid and cyprinid river spawners. The applicability of the assays was confirmed in a series of laboratory and field feeding experiments involving eggs and fish tissue. In addition, this work provides a guiding framework for conservation managers regarding the use and applicability of different DNA‐based detection approaches for gut content analysis.
  5. The results of this study could inform local conservation measures – such as temporary reductions in the density of round goby at spawning sites prior to spawning – and demonstrate how targeted application of species‐specific molecular markers may advance freshwater fish management.
  相似文献   

18.
Migration timing, speed, survival and effects of environmental parameters on migration, between wild and hatchery produced Atlantic salmon, Salmo salar L., smolts in the River Lærdalselva were studied. Hatchery‐reared (n = 40) and wild pre‐smolts (n = 40) were tagged with acoustic tags, and an array of receivers along the migration route was deployed. In all, 77 and 85% of the fish from the two groups, respectively, were recorded as migrating smolts, that is, predation rate and/or numbers of fish opting to remain in the river were low. Hatchery‐reared smolts showed a migration pattern, speed and migration route similar to wild smolts, even though the time period between river release and onset of migration was relatively short. Both groups of smolt showed high migration speed through both the river and the fjord compared with other studies.  相似文献   

19.
The effects of restoration of impassable road culverts on the distribution of juvenile Atlantic salmon, Salmo salar L., were evaluated in seven small tributaries of the subarctic River Teno system, northernmost Finland/Norway. Restoration enabled the passage of juvenile salmon through the culverts and increased the distribution area of salmon parr in the seven streams by tens or hundreds of metres, depending on the natural slope of the tributary with a total of ≈1 km new area for ascending juveniles. Areas upstream of the culverts were colonised after varying number of years, mostly 2–3, following restoration. Age‐1 and age‐2 parr were the first salmon age groups entering the new territory after removal of the migration barrier. Although the restoration measures were conducted at the downstream outlet area of the culverts only, the connectivity was improved and increased the production area accessible to juvenile salmon. Such removal of migration barriers and securing habitat connectivity by passable culverts should be taken into account in environmental management strategies of river systems safeguarding the essential habitats of salmonid fish.  相似文献   

20.
Oceanographic and predation processes are important modulators of fish larvae survival and mortality. This study addresses the hypothesis that immature Norwegian spring‐spawning herring (Clupea harengus), when abundant in the Barents Sea, determine the capelin reproduction success through consumption of Barents Sea capelin (Mallotus villosus) larvae. Combining a hydrodynamic model and particle‐tracking individual‐based model, a realistic spatio‐temporal overlap between capelin larvae and predatory immature herring was modelled for the summer seasons of 2001–2003. Capelin larvae originating from western spawning grounds became widely dispersed during the summer season, whereas those originating from eastern spawning grounds experienced a rapid drift into the southeastern Barents Sea. Herring caused a 3% mortality of the capelin larvae population in 2001 and a 16% mortality in 2003, but the effect of predation from herring on capelin larvae was negligible in 2002. Despite a strong capelin larvae cohort and a virtual absence of predatory herring, the recruitment from the capelin 2002 year class was relatively poor from a long‐term perspective. We show that the choice of capelin spawning grounds has a major impact on the subsequent capelin larvae drift patterns, constituting an important modulator of the capelin larvae survival. Variation in drift patterns during the summer season is likely to expose the capelin larvae to a wide range of hazards, including predation from young cod, sandeel and other predators. Such alternative predators might thus have contributed to the poor capelin recruitment during 2001–2003, leading to the collapse of the capelin stock in the subsequent years.  相似文献   

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