Distribution and exploitation of Nile perch Lates niloticus in relation to stratification in Lake Victoria,East Africa

Stratification restricts habitable areas forcing fish to balance between favourable temperature and minimum dissolved oxygen requirements. Acoustic surveys conducted during the stratified and isothermal periods on tropical Lake Victoria indicated that stratification of temperature and dissolved oxygen (DO) affected vertical distribution of Nile perch. There was higher mean temperature (25.6 ± 0.5 °C) and lower DO (6.4 ± 1.8 mg/l) during stratified period compared to the isothermal period (mean temperature 24.9 ± 0.3 °C; mean DO 7.3 ± 0.6 mg/l). Higher mean densities of Nile perch were recorded in the coastal (0.44 ± 0.03) and deep (0.27 ± 0.02 g/m³) strata during the stratified compared to the isothermal season (coastal: 0.24 ± 0.01; deep: 0.12 ± 0.02 g/m³). In addition, Nile perch density in the upper 0–40 m depth layers in the coastal and deep strata increased by over 50% from the isothermal to the stratified season. Daily landings from 65 motorised fishing boats between October 2008 and September 2010 show higher mean catch (26.29 ± 0.17 kg/boat/day) during stratified compared to the isothermal (23.59 ± 0.15) season. Thermal stratification apparently compresses the habitat available to Nile perch and can potentially result in higher exploitation. Managers should evaluate the potential benefits of instituting closed seasons during the stratified period, and stock assessment models should take into account the seasonal niche compression.     

Examination of Factors Contributing to the Decline of the Yellow Perch Population and Fishery in Les Cheneaux Islands,Lake Huron,with Emphasis on the Role of Double-crested Cormorants

Double-crested cormorants increased exponentially in the Les Cheneaux Islands area during the 1980s and 1990s. The yellow perch fishery and population declined by the late 1990s and finally collapsed in 2000. Previous research confirmed that cormorants fed seasonally on perch. This analysis sought to use creel survey data and data from an annual gillnet collection to characterize the perch fishery and population during this time so as to explore if declines were a result of declining recruitment or increased mortality or both. Regression analysis explored six possible independent variables to account for yellow perch trends. Yellow perch abundance and its fishery declined throughout the Les Cheneaux Islands. Mean age declined which was consistent with a high mortality rate explanation. Yellow perch recruitment, as indicated by gillnet catch rate of age-2 perch, continued during this time including one very strong year class. Total annual mortality rates determined by the cohort method were as high as 85% during much of this time and increased over the time series. Cormorant abundance accounted for a total of five significant relationships with the yellow perch data, more than any other independent variable. From this, it is apparent that cormorant predation is at least one factor affecting the perch population and fishery and may be the most influential force, among those examined, during this time series.     

Assessment of landed and non-landed by-catch of walleye, yellow perch and white perch from the commercial gillnet fisheries of Lake Erie, 1994-2007

Theoretical by-catch (including landed and non-landed bycatch) of walleye (S. vitreus), yellow perch (Perca flavescens), and white perch (Morone americana) from the Lake Erie commercial gillnet fisheries during 1994-2007, was predicted by a delta model developed on the fishery-independent survey data (Lake Erie Partnership Index Fishing Survey). The delta model consisted of one generalized additive model and one AdaBoost model. The generalized additive model was used to predict non-zero catches of the by-catch species, and the AdaBoost model was used to predict the probability of obtaining non-zero catches. Non-landed by-catch was estimated as the difference between the theoretical by-catch predicted from the delta model and the landed by-catch recorded in the commercial fishery data. The theoretical by-catch of walleye was relatively higher in the west basin in October. A higher theoretical by-catch of yellow perch occurred in the west central basin in November, and a higher theoretical by-catch of white perch occurred in the west central basin in October. We observed higher levels of non-landed by-catch of walleye in the west basin during August to September, higher levels of non-landed by-catch of yellow perch in the west central and east central basins in November, and higher levels of non-landed by-catch of white perch in the west basin in August and November. The combination of the AdaBoost model with the delta model provided an alternative model in by-catch analyses when the percentage of zero observations was high.     

Removal of brown trout (Salmo trutta L): Changes in population dynamics in a weir basin in Western Norway

The trout population in a 10.6 ha weir basin in western Norway has been studied over thirteen years. Baseline investigations were carried out during 1974–78, three to five years after construction of the weir. Studies in 1983 showed dominance of the 1977 and 1978 year-classes. This dominance lasted until 1985. Removal of 75 per cent of the population for stocking in reservoirs reduced the fish biomass from 1.43 to 0.28 kg 100 m^?2. This increased the growth rate and condition factor of the remaining fish in subsequent years. The trout enter the basin mainly at age 2+ and 3+. No increase in the number of immigrants was observed during the first two years after removal of fish. In the third year, however, a five-fold increase was measured. Most of these fish belonged to the 1985 year-class, which had little contact with the strong classes of 1977 and 1978. We conclude that density-dependent factors control immigration to the basin, whereas a combination of density-dependent and independent factors act on the recruitment in the riffle areas. We suggest that the highest recruitment of fish is obtained by reducing the density-dependent effect through removing a very high proportion of the young fish, 2+ and 3+, every year.     

Viral hemorrhagic septicemia virus infection in Yellow perch, Perca flavescens, in Lake Erie

Viral hemorrhagic septicemia virus (VHSV) infects wild and hatchery fish in Europe, Japan, and the Great Lakes and Pacific regions of North America. The virus was associated with a large die-off of yellow perch, Perca flavescens, in Lake Erie in 2006. To determine the infection pattern of VHSV, we sampled yellow perch during the spring, summer, and fall of 2007 and 2008 in the central basin of Lake Erie during routine sampling by the Ohio Division of Wildlife with bottom trawls in nearshore, mid-depth, and offshore locations near the Chagrin River. The Ohio Department of Agriculture's Diagnostic Laboratories and the U.S. Fish and Wildlife Service's La Crosse Fish Health Center tested for VHSV from homogenized samples obtained from yellow perch kidney, spleen, and brain. At each lake sample location, we also measured temperature, dissolved oxygen, and conductivity. In both years, we found yellow perch infected with VHSV during a three-week period starting in the last week of spawning to early June. A high proportion of adult male and female yellow perch tested positive for VHSV during the infection period in our sample population. Infection appeared to be associated with temperatures between 12 and 18 °C and with significantly higher yellow perch densities during spawning. No large mortalities of yellow perch were observed during the VHSV infection period in 2007 and 2008.     

Management effects on breeding and foraging numbers and movements of double-crested cormorants in the Les Cheneaux Islands,Lake Huron,Michigan

The yellow perch fishery of the Les Cheneaux Islands (LCI) region of Lake Huron, MI suffered a collapse in 2000, attributed in part to the increase of double-crested cormorants (Phalacrocorax auritus) in the region. A management program involving egg-oiling and lethal culling was initiated in 2004 to reduce cormorant foraging on yellow perch in the LCI. Counts of cormorant nests, nests oiled, cormorants culled, and aerial counts and telemetry surveys were used to evaluate management. Management contributed to a 74% reduction of cormorants on breeding colonies from 2004 to 2007. Cormorants used the LCI area significantly more (P < 0.05) than surrounding areas. Aerial counts of foraging cormorants declined significantly (P < 0.05) over the entire survey area but not within the LCI proper. However, aerial counts of cormorants in the LCI were five-fold less than cormorant counts in the same area in 1995. Reduced cormorant numbers were attributed in part to the elimination of cormorant nesting on a large colony due to the introduction of raccoons. Although the numbers of cormorants using the LCI did not decline, flocks were significantly smaller and more dispersed after management began. The reduced number of cormorants from 1995 levels and more dispersed foraging likely reduced predation on fish stocks including yellow perch in the LCI. Our findings indicate that the relationship between reduction in cormorant breeding numbers and reduced cormorant foraging in a given area is complex and may be influenced by density dependent factors such as intraspecific competition and quality of the forage base.     

Evaluation of anal fin spines,otoliths, and scales for estimating age and back-calculated lengths of yellow perch in southern Green Bay

Southern Green Bay supports important fisheries for yellow perch Perca flavescens and valid estimates of age structure and growth are critical to effective management. Anal fin spines and scales are used by the Wisconsin Department of Natural Resources for age estimation, but these structures may provide lower precision and accuracy than otoliths. The primary objective of our assessment was to determine if age estimates, among-reader precision, and mean back-calculated total lengths (TLs) at age differed among scales, anal fin spines, and otoliths. Ages estimated from anal fin spines were more precise than scale ages, were as precise as otolith-based ages, and generally agreed with consensus ages estimated from sectioned otoliths. Relationships between TL and radii of calcified structures were linear for scales, anal spines, and otoliths along two different transects. Mean back-calculated TLs were generally similar between intercept-corrected direct proportion (ICDP) and linear regression (LR) models, but otolith-based direct proportion models (no intercept correction) generally provided higher back-calculated mean TLs at ages 1 and 2 than ICDP and LR models. Mean back-calculated TLs at age estimated from whole otoliths were higher than estimates for other structures; but differences among anal fin spines, scales, and sectioned otoliths were <10?mm. Our results suggest biologists have little to gain by switching to otoliths when assessing age structure and growth for this fast-growing yellow perch population with relatively few fish ≥age 6, but additional analyses are warranted for slower-growing perch populations in the Great Lakes where older fish are more common.