Catchable Size Research Articles

The exploitation status and spawning potential ratio of blue swimming crab (Portunus pelagicus) in North Natuna Sea, Indonesia

Abstract Blue Swimming Crab (BSC) has been exploited in various areas, including the North Natuna Sea, because of its high selling value and potential as an export commodity. Overexploitation can threaten its sustainability. Therefore, information is needed regarding population status and spawning potential, particularly in the North Natuna Sea. This research was performed over three years, from April to December 2018, February to November 2019, and March to December 2020. A total of 4960 individuals of BSC samples ranged in size from 60 to 160 mm, with 115 mm being the most dominant. BSC exploitation in this water has reached E = 0.6, indicating that it has been overfished. The SPR value obtained was 19 percent. Even though there is high fishing pressure, the spawning potential of crabs is relatively high. Appropriate management policies are needed to maintain their spawning potential, including limiting the catchable size to a minimum of 60 grams and a policy of closing spawning locations.

Stock Assessment of the Commercial Small Pelagic Fishes in the Beibu Gulf, the South China Sea, 2006-2020.

Long-term variations in population structure, growth, mortality, exploitation rate, and recruitment pattern of two major commercial small pelagic fishes (CSPFs) (Decapterus maruadsi and Trachurus japonicus) are reported based on bottom trawl survey data collected during 2006-2020 in the Beibu Gulf, South China Sea. All individuals collected during each sampling quarter over a period of 15 years were subjected to laboratory-based analysis. In this study, the stock of D. maruadsi and T. japonicus inhabiting the Beibu Gulf was assessed using length-based methods (bootstrapped electronic length frequency analysis (ELEFAN)) to complete stock assessment in different fishery management periods (the division of fisheries management periods was based on China's input and output in the South China Sea offshore fisheries over 15 years, specifically divided into period I (2006-2010), period II (2011-2015), and period III (2016-2020)). The results showed that the mean body length, dominant body size, and estimated asymptotic length of two CSPFs decreased, whereas their growth coefficient decreased, indicating miniaturization and slower growth, respectively. Estimated exploitation rates and catching body length for two CSPFs indicated that both stocks in the Beibu Gulf were overexploited in period I and moderately exploited after 2011. These stocks were taking a good turn in status in period III, with the exploitation rate much lower than the initial period and reversing the downward trend in catching body length. Furthermore, the variations in the spawning season of the two CSPF stocks and their barely satisfactory expected yield indicated the complexity of the current fishery management in the Beibu Gulf. These results suggest that management measures to reduce fishing pressure may have a positive influence on the biological characteristics of those CSPFs in the Beibu Gulf; however, the stock structure already affected by overfishing will be a huge challenge for the conservation and restoration of fisheries resources in the future. Given that the current stocks of D. maruadsi and T. japonicus in the Beibu Gulf still have low first-capture body length (Lc) and high fishing mortality (F) (compared to F0.1), we identify a need to refine population structure by controlling fishing efforts and increasing catchable size, and more consideration should be given to the local fishery resource status in fisheries management.

This is what we know: Assessing the stock status of the data-poor skipjack tuna (Katsuwonus pelamis) fishery in the South China Sea

Skipjack tuna (Katsuwonus pelamis) is a highly migratory species of significant value in global marine fisheries. The South China Sea (SCS) is the largest marginal sea in the northwestern Pacific Ocean, and many tuna and tuna-like species occur within it. Although a recent Western and Central Pacific Fisheries Commission report identified skipjack tuna in the western and central Pacific to be sustainably fished, the exploitation status of skipjack tuna in the SCS is still unclear, due to limited data. We apply two data-poor methods, length-based Bayesian biomass estimation (LBB) and length-based spawning potential ratio (LBSPR), to assess the status of skipjack tuna stock in the SCS. We use electronic length frequency analysis to estimate von Bertalanffy growth parameters (asymptotic lengthLinfand growth coefficientK) to serve as priors for LBB and LBSPR estimation. Estimates are calculated for SCS skipjack tunaLinf(68.3 cm),K(0.325 year−1), natural mortality (0.49 year−1) and mean fork length at 50% sexual maturity (36.7 cm). LBB analysis reveals the estimated relative stock size (0.29) indicates that the SCS skipjack tuna stock maybe heavily overfished. LBSPR analysis reveals the estimated spawning potential ratio for this fishery to be 3%, which is significantly below the limit reference point of 20%. Accordingly, for the SCS skipjack tuna stock we identify a need to reduce fishing mortality by controlling fishing effort and increasing catchable size.

Do Cormorants and Recreational Anglers Take Fish of the Same Species and Sizes?

The Great Cormorant Phalacrocorax carbo is a widespread piscivorous waterbird. The competition for resources between recreational anglers and Cormorants has been causing serious conflicts between sport fisheries and environmentalists. This study aimed to compare fish catches by Cormorants and by recreational anglers in the upper Elbe River in Central Europe (Czech Republic). Cormorant diet was studied using regurgitated pellets, and catches of anglers were obtained from annual angling reports. Altogether 1478 Cormorant pellets were collected from which 6903 fish were measured and identified to species level. A total of 93,413 fish caught by anglers were identified to species level. Cormorant diet consisted of 24 fish species in six fish families. Cormorants caught smaller-sized fish (median mass 90 g) compared to fish caught by anglers (median mass 1700 g). The majority of fish caught by Cormorants were under the minimum legal catchable size for anglers. Species of moderate interest to anglers (mainly Roach Rutilus rutilus) dominated in Cormorant diets while Common Carp Cyprinus carpio dominated in catches of anglers. In conclusion, the direct competition for fish between anglers and Cormorants appeared low. However, as Cormorants consumed small fish that serve as prey for piscivorous fish species and that could potentially grow into legally sized fish for angling purposes, this still allows for indirect competition between Cormorants and sport fisheries.

Stock Assessment of the Blue Swimming Crab Portunus pelagicus (Linnaeus, 1758) in Stock Enhancement Sites of Danajon Bank, Central Philippines

The stock status of Blue Swimming Crab (BSC), Portunus pelagicus, locally known as lambay, in Danajon Bank was assessed from June 2019 to September 2020 to monitor the impact of the BSC stock enhancement initiative by the Philippine Association of Crab Processors, Inc. (PACPI) in collaboration with BFAR 7. The stock was assessed based on the indicators, namely, catch per unit effort (CPUE), exploitation rate (E), average size, length at first maturity (Lm), and Spawning Potential Ratio (SPR). Data analysis was done using FAO-ICLARM Stock Assessment Tools (FiSAT) II software and the Barefoot Ecologist’s Toolbox for the Length-based Spawning Potential Ratio (SPR). Results show that BSC dominated the monitored landings accounting for 66% of the overall catch harvested mainly by crab pot and crab net. Higher CPUEs were obtained in the months of July, June, May and August with annual mean observed to be higher than the previous assessment conducted by ECOFISH in Danajon Bank particularly for crab pot. Population characteristics revealed that male crab grew slightly faster having higher asymptotic length (L∞ =21.65cm Carapace Width or CW) and growth constant (K=1.3yr-1) than female (L∞ =21.40cm CW, K=1.28yr-1). Sex ratio of male to female was at 1:1.28 indicating plenty of female crabs were captured than male. Gravid crabs were abundant in May, December, April and October. Recruitment pattern showed two pulses with primary peak occurred in April and May and secondary pulse happened in August and September. Encouraging performance indicator noted from this assessment was on the bigger probability of capture sizes and mean length of commonly caught BSC falling above the recommended catchable size of 10.2 cm CW pursuant to DA-DILG JAO No. 1 s. 2014. Maturity size (Lm) was estimated at 10.54 cm CW, though smaller than the previous reported Lm values in Danajon but still within the threshold. The estimated SPR was 26%, higher than 20% limit reference point but slightly lower than 27% recorded in the assessment done by ECOFISH Project in 2014 to 2015. The present SPR though lower than 30% precautionary threshold however higher than 21% generated in Visayan Sea Region 7 for the same period. The higher SPR in Danajon Bank compared to Visayan Sea maybe credited to the effect of stock enhancement activity implemented in the area, though observation is not yet conclusive. On the other hand, unfavorable indicators were seen on high exploitation rate (E), high exploitation at yield per recruit some of which already breached the optimum level, consequently subjecting Danajon Bank to high fishing pressure. It is recommended to continue the BSC stock enhancement program of PACPI and BFAR 7 taking consideration some strategies for improvement specified in the recommendations.

Length distribution, gonad maturity level, and catchable size of fish caught around FADs in Ternate Sea

Pelagic fishery business in Ternate Sea are mainly based on FADs fishing operations. Increase of intensive fishing activities will also increase the pressures on local pelagic fish resources of Ternate Sea in the future. So far, there has been limited scientific information related to the fish caught by local fishers. The aim of the study was to investigate length distribution, gonad maturity level (GML), and catchable size of fishes caught around FADs. The research was conducted in August 2020 in Ternate. The research method was a survey research by collecting samples of fish caught from gill nets and hand lines for total 30 fishing trips. The study results showed that the catch consisted of mackerel scads and yellow scads. The mackerel scads caught by gill nets were between 13-26 cm in size and dominated by the 19-20 cm size class, while fishes sizes caught by hand line were between 14-27 cm which were dominated by the 18-19 cm size class. Yellow scads caught with both gill nets and handlines showed the same length range of 12-23 cm, which was dominated by the size class of 18-19 cm. Mackerel scads caught with gill nets and handlines were dominated by fish in GML II (47%) followed by GML III (32%). GML of yellow scads caught with gill nets was dominated by GML III by 75%, while for fishes caught by hand line the percentage of GML III is 68%. The mackerel scads caught with gill nets and handlines were dominated by the non-catchable size category (100% for gill nets and 98.79% for handlines). On the other hand, all yellow scads caught with both gill nets and handlines are entirely (100%) of catchable size.

Maturity Assessment for the Implementation of the First Fishery Regulation in Patagonian Marine Gastropods

Fil: Cumplido Esmoris, Mariano Jose. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Centro Nacional Patagonico. Instituto de Biologia de Organismos Marinos; Argentina

The Effect of Raceway Grading on Return to Creel for Catchable‐Sized Hatchery Rainbow Trout

AbstractHatchery trout of catchable size (i.e., about 250 mm total length; hereafter termed “catchables”), which are stocked into put‐and‐take fisheries, are expensive to raise, so fisheries management agencies strive to modify rearing practices to maximize the proportion of stocked catchables that anglers catch. We graded fish in production‐level hatchery rearing units at both fingerling and catchable size, dividing the fish into “leaders” (herein, the larger fish in a rearing unit, separated during a grading event) and “laggers” (the smaller fish). We hypothesized that (1) grading and immediate stocking of catchable leaders and short‐term retention and continued feeding of catchable laggers might increase lagger growth and result in a larger average size of fish being stocked (relative to fish from ungraded raceways), potentially improving overall return to creel for the graded group and (2) grading and separation of fingerling leaders and laggers early in the rearing process may reduce competition between smaller and larger fish for the remainder of the rearing period, potentially improving overall poststocking return to creel for the entire group without the need for catchable grading at the time of stocking. We found that grading catchables just prior to stocking slightly increased the mean size at stocking (compared with ungraded control fish) for the first stocking period, which resulted in slightly higher return to creel for stocked fish. However, across the entire stocking period and with equal feed between groups, mean size at release was nearly identical between the graded and the control fish, as was return to creel by anglers. Grading fingerlings to separate leaders and laggers for the remainder of the rearing period also had no positive effect on angler catch. Our findings suggest that production‐level grading is ineffective at increasing the growth of laggers, so it will not increase overall size at stocking. Consequently, improvements in angler catch are unlikely to materialize from grading hatchery trout prior to stocking them into put‐and‐take fisheries.

Estimating the catchable size of orange-spotted grouper (Epinephelus coioides) in Kwandang Bay, Gorontalo Utara District, Indonesia

The orange-spotted grouper (Epinephelus coioides) is a valuable food fish. To support the sustainable management of orange-spotted grouper fisheries, it is important to determine the optimum catchable size. The purpose of this study was to determine an optimal catchable size for the orange-spotted grouper stock in Kwandang Bay, North Gorontalo District, Indonesia. The Catchable size was determined based on the size at first sexual maturity, fecundity, and reproductive potential. The study was conducted from December 2016 to November 2017. The size at first sexual maturity was estimated based on the examination of gonads from 141 specimens, while fecundity was estimated based on a sample of 40 female fish with ripe gonads. The mean total length (TL) at first maturity was 40 cm. Fecundity ranged from 30,526 - 1,395,846 eggs with a mean of 687,025 eggs. The reproductive potential was highest in the size range of 55-64 cm TL. The recommended size for orange-spotted grouper is above 64 cm, to enable females to attain their maximum level of egg production before capture and thus contribute to maintaining the stock.

How ecological processes shape the outcomes of stock enhancement and harvest regulations in recreational fisheries.

Fish stocking and harvest regulations are frequently used to maintain or enhance freshwater recreational fisheries and contribute to fish conservation. However, their relative effectiveness has rarely been systematically evaluated using quantitative models that account for key size- and density-dependent ecological processes and adaptive responses of anglers. We present an integrated model of freshwater recreational fisheries where the population dynamics of two model species affect the effort dynamics of recreational anglers. With this model, we examined how stocking various fish densities and sizes (fry, fingerlings, and adults) performed relative to minimum-length limits using a variety of biological, social, and economic performance measures, while evaluating trade-offs. Four key findings are highlighted. First, stocking often augmented the exploited fish population, but size- and density-dependent bottlenecks limited the number of fry and fingerlings surviving to a catchable size in self-sustaining populations. The greatest enhancement of the catchable fish population occurred when large fish that escaped early bottlenecks were stocked, but this came at the cost of wild-stock replacement, thereby demonstrating a fundamental trade-off between fisheries benefits and conservation. Second, the relative performance of stocking naturally reproducing populations was largely independent of habitat quality and was generally low. Third, stocking was only economically advisable when natural reproduction was impaired or absent, stocking rates were low, and enough anglers benefitted from stocking to offset the associated costs. Fourth, in self-sustaining fish populations, minimum-length limits generally outperformed stocking when judged against a range of biological, social and economic objectives. By contrast, stocking in culture-based fisheries often generated substantial benefits. Collectively, our study demonstrates that size- and density-dependent processes, and broadly the degree of natural recruitment, drive the biological, social, and economic outcomes of popular management actions in recreational fisheries. To evaluate these outcomes and the resulting trade-offs, integrated fisheries-management models that explicitly consider the feedbacks among ecological and social processes are needed.

Biological characteristics, temporal-spatial distribution of Portunus trituberculatus and relationships between its density and impact factors in Laizhou Bay, Bohai Sea, China.

Biological characteristics, density and its spatial distribution of Portunus trituberculatus in Laizhou Bay, together with the impact of environmental factors, were analyzed based on the bottom trawl data of 9 surveys from May 2011 to April 2012, in order to provide scientific basis for utilizing and protecting the resource of P. trituberculatus. The monthly variation of biomass density could be described as September > October > July > August > June > November > March > May > April. The average values were highest in August 2011 and lowest in April 2012, for body mass, width and length, respectively. The individual fatness coefficient was highest in August 2011 both for the female (1.030) and the male (1.023), lowest in July both for the female (1.007) and the male (1.007). The difference of fatness coefficient was not significant (P>0.05) between the female and the male. The sex ratio (female/male) was less than 1.0 in most of all months except June, July and August. P. trituberculatus was mainly distributed in the Yellow River estuary and the offshore off Longkou in May-July, in offshore off Weifang and Longkou in August-September, and in deep-water area of the Laizhou Bay mouth from October to next April. Pearson correlation analysis indicated that environmental factors that best matched the density of P. trituberculatus were sea surface temperature (SST), dissolved oxygen (DO) and water depth (Dep), followed by salinity (Sal) and zooplankton density, the last were phytoplankton density and the number of other demersal fisheries species. The body length of P. trituberculatus in 2011 was significantly smaller (P<0.01) compared with that in 1981. To sustainably utilize the resource of P. trituberculatus in Laizhou Bay, we suggested that the fishing effort should be reduced, the catchable size should be raised, the research of enhancement basements should be promoted and more reasonable releasing quantity should be proposed.

Status of Redband Trout in the Upper Snake River Basin of Idaho

AbstractRedband Trout Oncorhynchus mykiss gairdneri are likely the most abundant and most widely distributed native salmonid in the Columbia River basin, yet their current distribution and abundance across the landscape have not been well documented. We sampled 1,032 randomly distributed stream sites (usually 100 m in length) across more than 60,000 km of stream network to assess Redband Trout occupancy, abundance, and genetic purity in the upper Snake River basin of Idaho. Study locations were more often in dry desert subbasins (49% of sites) than in montane subbasins (20%), and 25% of the dry “stream sites” had no discernible stream channel whatsoever, indicating a lack of flowing water for perhaps millennia. Redband Trout were estimated to occupy 13,485 km of stream (22% of the total) and were captured more often (389 sites) than Brook Trout Salvelinus fontinalis (128 sites), Bull Trout Salvelinus confluentus (37 sites), or Brown Trout Salmo trutta (16 sites). Redband Trout were also the most abundant species of trout, with an approximate abundance of 3,449,000 ± 402,000 (90% confidence interval) of all sizes, followed by Brook Trout (1,501,000 ± 330,000), Bull Trout (159,000 ± 118,000), and Brown Trout (43,000 ± 25,000). Approximately 848,000 ± 128,000 Redband Trout were adults. From 1913 (the earliest year of record) to 2001, roughly 43 million hatchery Rainbow Trout were stocked in streams in the study area, 17.5 million of which were of catchable size (i.e., ≥200 mm total length); since 2001, all catchable trout have been sterilized prior to stocking. Genetic results from 61 study sites suggest that hybridization with hatchery Rainbow Trout is more likely to occur in streams that were directly stocked with catchable trout from 1913 to 2001. Applying these results across the landscape, we estimated that Redband Trout likely remain pure in about 68% of the streams occupied in the upper Snake River basin.Received November 13, 2012; accepted January 9, 2014

Effects of Stocking Catchable‐Sized Hatchery Rainbow Trout on Wild Rainbow Trout Abundance, Survival, Growth, and Recruitment

AbstractThe Idaho Department of Fish and Game has proactively dealt with the potential adverse genetic effects of stocking catchable‐sized hatchery trout in waters that support native salmonids by adopting a policy in 2001 whereby only sterile rainbow trout Oncorhynchus mykiss are stocked in flowing waters; however, concerns regarding the competitive effects of introducing hatchery trout into streams and rivers supporting wild trout have not been addressed. We stocked fish in the middle 3 years of a 5‐year study to assess whether stocking hatchery rainbow trout of catchable size (hereafter, catchables) reduced the abundance, survival, growth, or recruitment of wild rainbow trout in streams. Catchables averaging 249 mm total length (TL) were stocked from 2006 to 2008 at an annual density of 4.2 fish/100 m2 into 12 treatment reaches of stream that were paired with control reaches at least 3 km apart in the same stream in which no stocking occurred. Wild rainbow trout abundance (including all fish ≥75 mm TL), recruitment, survival, and growth were determined from population estimates and recaptures of fish tagged with passive integrated transponder (PIT) tags during mark–recapture electrofishing sampling. The abundance of wild rainbow trout averaged 13.2 fish/100 m2 but varied substantially across sites and years, ranging from a low of 0.5 to a high of 131.3 fish/100 m2; similar variability was observed in recruitment to age 1. Estimates of total annual survival averaged 0.53 based on the population abundance estimates (which allowed for emigration and immigration) and 0.26 based on the PIT‐tag recaptures (which allowed for emigration but not immigration). Our paired study design demonstrated that the abundance, survival, growth, and recruitment to age 1 of wild rainbow trout were all unaffected by stocking catchables. The lack of population‐level effects from stocking catchables was not surprising considering the high short‐term mortality and the socially and physiologically naive behavior typically exhibited by hatchery catchables stocked in lotic systems.

Effect of supplementary stocking of juvenile brown trout,Salmo trutta, on yield in a Norwegian mountain reservoir

The effect of supplementary stocking of juvenile (age 0+), hatchery-reared, brown trout, Salmo trutta L., on annual yields was assessed in a Norwegian mountain reservoir between 1979 and 2007. Fishing was mainly carried out by local fishermen with benthic gillnets. During the study period, annual stocking ranged from 0 to 52 500 fish (19.8 ha−1). No stocking has been carried out since 1997. Annual yield varied from 1650 to 5653 kg, corresponding to 0.62–2.13 kg ha−1. Exploitation rate in terms of number of gillnets and mean weight of 6+ fish (age when catchable size was reached) explained 64% of the variability in catches. Stocked fish contributed very little to the yield or catch-per-unit-effort, exhibiting no positive correlation with stocking density. The lack of contribution from stocked fish was probably caused by a competitive bottleneck in the eroded epibenthic zone, causing high juvenile mortality. If stocking continues, it is recommended that fish with body lengths >15–20 cm are used.

Survival and Dispersal of Hatchery Triploid Rainbow Trout in an Idaho River

AbstractHatchery rainbow trout Oncorhynchus mykiss of catchable size (hereafter, “catchables”) are commonly released into streams to improve angler catch and harvest rates, but returns are often less than 50% and the fate of unharvested catchables is largely unknown, especially for sterilized hatchery trout. The survival and dispersal of triploid catchables were investigated using snorkel and telemetry techniques to quantify the persistence and dispersal of catchables stocked three times during the summer of 2006 (June, July, and August) into an Idaho river in which a minimum length regulation precluded harvest. Counts of catchables stocked with T‐bar tags (n = 900) and catchables emitting active radio transmitter codes (n = 54) steadily declined throughout the three observation periods. Dispersal of T‐bar‐tagged catchables 30 d poststocking was generally downstream of the stocking point (median dispersal = 100 m). The median values of the maximum known downstream and upstream dispersal distances for catchables with radio transmitters were 5.0 and 1.2 km from the stocking point, respectively. Only four transmitter‐implanted catchables were still alive on November 1, when the study ended. On average, 85% of catchables with radio transmitters were presumed dead at 30 d poststocking (average persistence = 14.3 d). A similar rate of decline was observed for T‐bar‐tagged catchables. Managers wishing to maximize return‐to‐creel rates of sterile catchables in lotic systems might do so by limiting stocking events to within 3 weeks of expected needs and limiting the stocking locations to within 1 km of areas frequented by anglers.

Hydroacoustic assessment of fish resources in the Lake Qinghai with EY60 echosounder

2006年8月16-18日通过使用回声探测仪(Simrad公司EY60型,200kHz换能器)对青海湖的鱼类资源进行了探测评估.结果表明:青海湖裸鲤(Gymnocypris przewalskii)水平分布具不均匀性,有小型鱼群聚集现象;在不同的设定分区,鱼类密度在0.168-12.8ind./1000m³之间,均值为1.16ind./1000m³;布哈河口鱼类较为密集,密度最大值为12.8ind./1000m³.初步估算青海湖裸鲤达可捕规格(体长大于20cm)个体数量为6.5×10⁷ind.,95%置信度区间为35.6×10⁷-118.4×10⁷ind.,应用资源密度体积法估算青海湖裸鲤的可捕资源量约为15275t.探测实验还证实,EY60回声探测仪在青海湖中进行鱼类资源评估时探测效果良好,分析软件Sonar5可以对鱼类个体自动识别、计数,便于进行鱼类资源量评估.;A SIMRAD EY60 echo-sounder with 200kHz split-beam transducer was used to survey the fish school in LakeQinghai on August 16-18, 2006. The transducer was tied on the side of the ship downward at one meter depth. The cruising speed is8-10km/h. Also, a GPS was connected with EY60 echo-sounder for obtaining the survey position data. The pulse duration was setat 64μs. At post-processing, the Sonar5-pro software (versions 5.9.6) was used to analyze the echo-sounder data. The results show that the Gymnocypris przewalskii is not uniform distribution in horizontal direction in Lake Qinghai, and appears the feature of smallaggregation. For different survey subarea, the density of fish is 0.168-12.8ind./1000m³, and the average is 1.16ind./1000m³where themaximum appeared near the Buha estuary. The biomass of catchable size is estimated at 6.5×10 ⁷ ind. near 15275 ton, at35.6×10⁷ -118.4×10⁷ind. with 95% confidence interval. Our results also indicated that the EY60 echo-sounder is suitably applicablein Lake Qinghai to estimate the fish productions effectively.

Time-series analysis of abundance indices of young fish

Abstract Catch-at-age analysis provides estimates of stock size at ages when the fish have reached catchable size. Survey indices contain information about relative cohort size at younger ages. The present analysis is concerned with survey indices of juveniles up to the youngest age where stock estimates, based on time-series analysis of catch-at-age data, are available. A stock estimate at that age from catch-at-age data is also included. A common model of the relationship between stock size and survey indices is combined with the model describing the decline of a stock by natural mortality. Random variations in natural mortality are defined separately from sampling variations and irregular catchability in the survey. The stock size and magnitudes of the random variations are estimated by the Kalman filter, which also provides predictions of future recruitment to the catchable stock. Analysis of observations of Icelandic cod reveals a large deviation from proportionality in the relationship between the index and the stock estimates in the youngest ages, but haddock data are compatible with proportionality. Variations in natural mortality during the second to fourth year of cod and the second to third year of haddock are not a major factor in variations of stock size.

Relationship between Spring Snow Depth and Growth of Brown Trout, Salmo trutta, in an Alpine Lake: Predicting Consequences of Climate Change

Growth of brown trout in the alpine lake Litlosvatn, 1172 m a.s.l. on the Hardangervidda mountain plateau in western Norway, has been studied in relation to annual variations in accumulated spring snow and date of ice-breaks during the period 1991–1998. The annual growth was negatively correlated with the spring snow depth. During the years 1992–1995, a period with mean spring snow depth at 275 cm, the observed average annual growth of the age-classes 6–8 was 3.5 cm, or a reduction of about 50% compared to years with much less snow in spring (1991 and 1996). Average spring snow depth during the period 1990–1999 was 220 cm, while the average for the period 1930–1979 was 169 cm. A further increase in winter precipitation, resulting in even more accumulated spring snow compared to the 1990–1999 level, would result in further reduction in growth of brown trout in the lake. Thus, the present catchable size of brown trout would probably not be reached, due to increased population density and an additional density dependent growth. A climate change, resulting in more snowfall in the whole alpine region of West Norway, as predicted by climate models, may therefore have an important negative effect on fish production and fisheries in a whole region.

Relationship between Spring Snow Depth and Growth of Brown Trout, Salmo trutta, in an Alpine Lake: Predicting Consequences of Climate Change

Growth of brown trout in the alpine lake Litlosvatn, 1172 m a.s.l. on the Hardangervidda mountain plateau in western Norway, has been studied in relation to annual variations in accumulated spring snow and date of ice-breaks during the period 1991–1998. The annual growth was negatively correlated with the spring snow depth. During the years 1992–1995, a period with mean spring snow depth at 275 cm, the observed average annual growth of the age-classes 6–8 was 3.5 cm, or a reduction of about 50% compared to years with much less snow in spring (1991 and 1996). Average spring snow depth during the period 1990–1999 was 220 cm, while the average for the period 1930–1979 was 169 cm. A further increase in winter precipitation, resulting in even more accumulated spring snow compared to the 1990–1999 level, would result in further reduction in growth of brown trout in the lake. Thus, the present catchable size of brown trout would probably not be reached, due to increased population density and an additional density dependent growth. A climate change, resulting in more snowfall in the whole alpine region of West Norway, as predicted by climate models, may therefore have an important negative effect on fish production and fisheries in a whole region.

Relative Return to Creel of Triploid and Diploid Rainbow Trout Stocked in Eighteen Idaho Streams

Introductions of fertile nonnative hatchery trout have led to interspecific and intraspecific hybridization of native salmonid stocks throughout North America. Use of sterile triploid hatchery trout in stream-stocking programs could reduce genetic risks to native stocks while addressing public demand for consumptive fishing opportunity. Techniques to produce triploid salmonids are well developed, and triploid rainbow trout Oncorhynchus mykiss are readily available from commercial sources. However, there is no published information on the return to creel of triploid trout in stream recreational fisheries. We purchased mixed-sex triploid and diploid rainbow trout eggs from a commercial supplier and reared the resulting fish to catchable size. Flow cytometry was used to verify triploid induction rates in the triploid group. Estimated cost to produce a triploid catchable rainbow trout was about 15% higher than for a diploid fish. We jaw-tagged and stocked 300 triploid and 300 diploid fish into each of 18 streams throughout Idaho and used tag returns to assess relative return to creel and timing of returns for the two groups. In all, 1,849 tags were returned by anglers, 931 from triploid fish and 918 from diploid fish. Overall returns were not significantly different between groups (paired t-test; P = 0.80). Mean time to harvest also did not differ between groups (paired t-test; P = 0.35). These results suggest that triploid rainbow trout can provide stream angling opportunity equal to that provided by fertile diploid fish. Although there are other concerns regarding the stocking of hatchery trout in streams containing native trout, we suggest that using triploid rainbow trout in stream-stocking programs can help balance the demands for both consumptive fishing opportunity and conservation of native stocks.