Strong Year-classes Research Articles

Does the benthic paradox tell us something about surplus production models?

It is paradoxical that the taxonomic composition and diversity of the benthic infauna of continental shelves is similar from tropical to polar seas, while the associated demersal fish communities are less diverse and taxonomically different at high latitudes. Further, benthic invertebrates at high latitudes deal with the unreliable seasonality of plankton production by brooding their eggs and larvae instead of releasing them into the plankton, while only exceptional fish make the same response. These facts, and the notoriously variable reproductive success of cold-water fish, suggest that reproduction may be the critical biological process for marine organisms at high latitudes. Some fish perhaps exploit the potential longevity of mature individuals to bridge long gaps between years of successful reproduction, and cannibalism on young fish of weak year-classes by survivors of a strong year-class may support sufficient longevity for this to occur. Surplus production models ignore this possibility.

Dispersal Patterns of Subadult and Adult Colorado Squawfish in the Upper Colorado River

Abstract We investigated distribution and dispersal patterns of subadult and adult Colorado squawfish Ptychocheilus lucius (recently renamed the Colorado pikeminnow) throughout their range in the upper Colorado River. Annual, river-wide, capture–recapture data were used to document movements during a 5-year period (1991–1995). Average total length of Colorado squawfish progressively increased upstream: juveniles and subadults occurred almost exclusively in the lowermost 105 km of the 298-km study area, whereas most adults were concentrated in the uppermost 98 km. This was most pronounced early in the study and less so later due to the effect of two or three strong year-classes that dispersed through the system. Only 16% of subadult and adult fish initially captured and tagged in the upper reach were later located more than 10 km from the previous capture site; of those tagged in the lower reach, 58% were later located more than 10 km from the previous site. Most movements greater than 10 km were directed ...

On the population development of the introduced razor clamEnsis americanus near the island of Sylt (North Sea)

The American razor clamEnsis americanus (=E. directus) was introduced into the eastern North Sea in the late 1970s. By larval and poslarval drifting the species rapidly extended its distribution, now ranging from the English Channel to the Kattegat. Near the island of Sylt in the eastern North Sea it has been recorded since 1979. Recruitment was rather irregular, with about six strong year-classes within two decades. Growth seems comparable with populations in its native range (Atlantic North America). Although present in the lower intertidal zone, maximum densities occurred in shallow subtidal sand with a biomass similar to that of dense beds of native cockles and mussels in the adjacent intertidal zone.Ensis americanus established in otherwise sparsely faunated sand (channels exposed to strong currents) as well as in dense infaunal assemblages (lower intertidal and subtidally). There were no significant interactions with resident species. In dense beds of razor clams, however, fine sediment particles accumulated which may have altered abundances of polychaetes. In spite of high annual variability,E. americanus has become a prominent component of the macrobenthos in shallow subtidal sands of the North Sea.

Larval Sampling as a Fisheries Management Tool: Early Detection of Year-Class Strength

Larvae of black crappies Pomoxis nigromaculatus, white crappies P. annularis, and white bass Morone chrysops were sampled in 1992–1996 from Normandy Reservoir, Tennessee, with a 1 × 2-m neuston net. Larval crappies were not captured in 1992 or 1993, but densities over the remaining three years varied over two orders of magnitude. Larval white bass were collected every year, but densities also varied over two orders of magnitude among years. Larval crappies recruited to the gear at 9 mm total length (TL), but few over 15 mm were collected. Larval white bass recruited to the gear at 7 mm TL and continued to be sampled by the neuston net at lengths up to 35 mm TL. Peak larval crappie density in the neuston net samples was an accurate predictor of geometric mean number of age-1 crappies per hectare 1 year later in midsummer cove samples (r2 = 0.99, P = 0.0001). Peak white bass density in the neuston net samples was an accurate predictor of geometric mean catch of age-0 white bass in fall gill-net samples (r2 = 0.77, P = 0.05). Larval sampling of these species over a few weeks each spring in Normandy Reservoir can accurately demonstrate the presence or absence of strong year-classes much earlier and with less effort than traditional sampling techniques such as fall gillnetting or cove sampling. Early detection of year-class strength via neuston-net sampling may allow managers to predict poor year-classes early in the year and initiate remedial actions such as supplemental stocking or regulation changes in a more timely manner.

Use of Egg Presence–Absence to Derive Probability-Based Management Criteria for Upper Chesapeake Bay Striped Bass

I used egg presence–absence for striped bass Morone saxatilis from five Maryland spawning areas during 1955–1992 to develop criteria for assessing whether upper Chesapeake Bay spawning stock was at a level providing high and stable recruitment or below a level where recruitment declines appreciably. Strong year-classes did not occur when the proportion of tows with eggs (Ep) fell below 0.60. and the probability of a poor year-class occurring increased greatly as Ep fell below 0.80. Mean recruitment increased and coefficient of variation decreased as Ep rose from 0.40 to 0.80. Highest mean recruitment and lowest coefficient of variation occurred when Ep was at or above 0.80. A probability-based assessment of upper Chesapeake Bay spawningstock recovery indicated a high risk that spawning-stock status did not recover in 1994 and a low risk in 1995. Upper Chesapeake Bay Ep, grew during the late 1950s, was highest and stable during the 1960s, decreased through the 1970s to its nadir in the mid-1980s, and then grew quickly through the early 1990s. Stable Ep coincided with annual instantaneous fishing mortality rates (F) of 0.50–0.60; decreasing Ep occurred when F exceeded 0.75, and increasing Ep occurred when F was less than 0.25.

Settlement timing, distribution, and abundance of Dover sole (Microstomus pacificus) on an outer continental shelf nursery area

Settlement timing, distribution, and abundance of Dover sole (Microstomus pacificus) were compared with depth and environmental variables off Oregon, U.S.A. Surveys were made every 2 months from January to November 1989 on three 50-400 m depth transects and every March from 1990 to 1994 on one transect. Annual March age-1 catch per unit effort (CPUE) estimates did not support Hayman and Tyler's (R.A Hayman and A. Tyler. 1980. Trans. Am. Fish. Soc. 109: 54-70) hypothesis that strong year-classes are associated with offshore convergence during the settlement season, despite a similar distribution of oceanographic conditions during the two studies. High March CPUE was correlated with high bottom temperature in the primary nursery area (100-119 m depth) and an increase in the proportion of larvae in deeper strata. Seasonal and annual observations support Hayman and Tyler's assumption that the primary postsettlement nursery area is bathymetrically restricted. Otolith increments indicated that larvae at 50-159 m had been on the bottom longer than larvae from greater depths, suggesting postsettlement inshore migration. Sediment size and bottom temperature do not explain the high CPUE in the primary nursery area. A negative size-depth relationship and a change in depth distribution with increasing density suggest that biological interactions may influence the spring distribution.

Growth and expansion of an exploited population of the squat lobster ( Pleuroncodes monodon) after 3 years without harvesting

In 1991, after 3 years of fishery prohibition, the most important surviving population of squat lobster ( Pleuroncodes monodon, Galatheidae, Decapoda) on the continental shelf of central Chile, the Achira population (36 °S, 73 °W), reached the highest biomass recorded since the start of population assessments in 1979 (34 084–43 910 t, 95% confidence interval), and expanded its distribution by 56 km to the north. The age composition of the population was characterised by two strong year-classes (1988 and 1989, born at the end of the previous fishing period and at the start of the prohibition respectively) and a mixture of older year-classes, less important in terms of numerical abundance. Population expansion had a clear spatial structure. The 1989 year class (mean size 23.5 mm carapace length) populated the traditional southern population area, while the 1988 and older year-classes (mean carapace length 33.5 mm and 39.5 mm, respectively) populated the areas to the north which were vacant prior to the fishery closure in 1989. We interpret this spatial structure as the result of size-dependent dispersal capacity, with recruitment of the 1989 year-class providing the impetus for expansion of the individuals of the 1988 and older year-classes. Using the simple age structure of the Achira population, we estimated an average rate of population increase of 10 323 t year −1 during the period without harvesting, and a rate of expansion of 20 km year −1. Most of the reproductive potential (eggs km −2) was transported into the expansion zone, which may create new nursery grounds towards the north. This in turn may alter the spatial distribution of size, if recruitment is successful in the northern areas, with a potential effect on expansion dynamics.

Annual variation in parental care in smallmouth bass,Micropterus dolomieu

An eight year field study is summarized outlining variation in parental care in a northern population of smallmouth bass. Nesting makes ranged in age from 4 to 12 + years old and ranged in size from 21 cm to 40 + cm in fork length. The age and size distributions were skewed right when strong year-classes entered the nesting stock. Larger makes had more offspring in their nests than smaller makes. The year-to-year variation in the duration of care was large and appears to be related to two general effects of water temperature. First, there was a significant positive relationship between the variation in spring spawning date and variation in the duration of parental care prior to swim-up of the larvae. This reflects the general effect of spring warming rate on the timing of nesting in each year. Second, in most years, there was a significant positive relationship between male size and the duration of parental care prior to swim-up. This reflects the consistent inverse relationship between male size and the seasonal timing of spawning in this population. The offspring of large makes spawning early in a season take longer to develop than offspring of makes nesting late in season. There is no relationship in most years between male size and duration of care after swim-up. Other factors unrelated to water temperature may be important in determining the duration of care after swim-up. In particular, ontogenetic changes in juvenile behaviour and energy depletion in nesters may be two important factors affecting care duration.

Piscivory, Growth, and Size-Selective Mortality of Age 0 Pikeperch (Stizostedion lucioperca)

Year-class strength of pikeperch (Stizostedion lucioperca), indexed as the age 0 abundance in trawl surveys, varied 300-fold in the northern part of Lake IJssel, The Netherlands, over the period 1966–89. Both mean length and year-class strength of age 0 pikeperch in November were highly correlated with mean summer temperature. Depending on the environmental conditions, especially water temperature and availability of food, the initially unimodal length frequency distribution of age 0 pikeperch developed into a positively skewed, bimodal or negatively skewed distribution towards the end of the summer. Strong year-classes were characterised by larger mean lengths and a negatively skewed frequency distribution, while weak year-classes were smaller and positively skewed. Stomach contents consisted of zooplankton and macrofauna for the smaller specimens and of age 0 smelt (Osmerus eperlanus) for the larger pikeperch (> 10 cm). Body energy content increased with fish length, and differences in proximate analysis were more pronounced later in the season. The condition of nonpiscivorous age 0 pikeperch was low and decreased over time, while that of piscivorous pikeperch increased. The onset of piscivory, favoured by high temperatures during summer, has a direct positive effect on the growth and survival of age 0 pikeperch.

Climatic Influence Linking Copepod Production with Strong Year-Classes in Sablefish, Anoplopoma fimbria

Stomach contents from first-feeding larval sablefish (Anoplopoma fimbria) comprised mainly calanoid copepods. Along the west coast of Vancouver Island, these copepods were the dominant zooplankton at the depth that sablefish larvae developed. We propose that strong year-classes in sablefish populations occur when there is exceptional production of copepods. The periods of exceptional copepod production appear to be correlated with climate and ocean conditions.

Validation of the use of Otoliths to determine the ate and growth of Murray Cod, Maccullochella peelii (Mitchell) (Percichthyidae), in Lake Charlegrark, Western Victoria

The age and growth of Murray cod, Maccullochella peelii, in Lake Charlegrark, western Victoria, were determined by using otoliths from fish sampled by beach-seine and gill-net, or caught by anglers, between August 1978 and August 1982. Annulus formation was validated up to 11 + years by several methods. Otoliths were examined from hatchery-bred fish of known age (up to 4+ years) that had been stocked into local farm ponds. Analysis of incremental growth for all fish sampled from the lake indicated that the mean edge-growth ratio was lowest, and the percentage of otoliths with an annulus at the edge was highest, in October. A nominal birth date of 1 October was designated for the population to enable calculation of the absolute age for each fish. Age-composition data for those fish taken by gill-net and by anglers (up to 21 + years) revealed modal progression of strong year classes (1970 to 1973 inclusive) during the sampling period. Length-frequency data for those fish taken by beach-seine (up to 3 + years) revealed modal progression, on a seasonal basis, of two relatively strong year classes (1978 and 1979) during the sampling period. A von Bertalanffy growth curve was fitted to length and age data for all fish collected in the lake (no sex difference, P>O.05), and growth parameters were estimated for the population (L∞ = 695 mm). This curve did not adequately describe the growth of fish older than 11 + years because the majority of sampled fish (98.3%) were aged 11 + years or younger. The length-weight relationship for the population was determined by using data from all fish collected in the lake (no sex difference, P>0.05).

Effects of year-class strength on age determination

Little has been done to assess factors that can affect precision and accuracy in the fish age determination process. In this study, we examine how the presence of strong year-classes (i.e. foreknowledge of these year-classes) can affect the ages that are generated. It has been suggested recently that reader precision (i.e. reader repeatability) places limits on the accuracy of ages and that reader awareness of year-classes could both increase estimated precision and exaggerate the strength of strong year-classes beyond that which could be expected considering reader repeatability. That is, if reader repeatability was 60% at a given age, then a maximum of 60% of any strong year-class of that age could be expected to be aged correctly. However, consciously or subconsciously, age readers can be expected to tend to age toward strong year-classes. In order to examine these hypotheses, an otolith test sample was constructed for walleye pollock (Theragra chalcogramma) by combining samples that were collected in 3 adjacent years and that were known to include the strong 1978 year-class. Thus, the test sample gave the appearance of consisting of three neighbouring age-classes in nearly equal numbers. The age readers involved in this study had results that were better than might be expected, maintaining their reader precision even with this difficult sample. However, the results also appear to indicate that the 'dominant' year-class was spread to other ages in comparison with ages obtained when the age readers were aware of the strong year-class.

Growth of Bream,Abramis brama L., in Two Outside Basins of Different Trophic State of Lake Balaton

Age-composition, size-distribution, length/weight relationships and growth both in length and weight of bream stocks were studied on fishes collected in NE and SW basins of Lake Balaton during 1982/83. Stocks consisting of 10 to 12 age-groups were observed at the two lake areas. The linear growth and weight-gain in the SW-basin were annually faster than in the NE-basin, however, the growth of bream in Lake Balaton was slow as compared to that observed 10 years ago. Parallel with progressive eutrophication, the density of bream stocks increased by some 5–10 % especially at hypertrophic lake areas (SW) and consequently their growth was transformed to densitydependent. In the appearance of strong year-classes some cyclicism could be observed especially in the SW-basin.

Temperature and Lobster, Homarus americanus, Yield Relationships

Commercial landings of American lobster (Homarus americanus) have increased dramatically in many areas along the east coast of Canada since the late 1970's. Average annual sea surface temperatures (SST) were analysed to test if lobster landings were related to changes in the marine environment. Time series models were fitted to lobster landings and SST data from Maine (USA), Halifax County (Nova Scotia), and Charlotte County (New Brunswick). Including temperature in the models improved forecasting ability for lobster catches for Maine and Halifax but not Charlotte. In Maine, lobster landings in year t were related to SST in year t. In Halifax, however, landings were related to SST in the previous 4 yr but not by SST of the current year. Lower fishing effort levels for Halifax compared with Maine probably extended the yield from strong year-classes through a number of years. We surmise that an increase in sea temperature near Halifax during the early 1980's increased the survival and growth rates of lobster juveniles. This provided a strong pulse of recruits to the fishery throughout coastal Nova Scotia during the mid- to late 1980's. Similar increases in temperature and landings were not observed for Maine and Charlotte.

Fluctuations in Year-class Strength of Japanese Spanish Mackerel in the Central Seto Inland Sea.

The abundance of larval and juvenile Japanese Spanish mackerel Scomberomorus niphonius in Hiuchi-nada, the central part of the Seto Inland Sea, and its adjacent waters, was investi-gated by net sampling from 1983 to 1988. The relationships between larval/juvenile abundance, CPUE of subsequent 1-year-old fish and oceanographic conditions were examined. There was a significant, positive correlation between the abundance of juveniles and the CPUE of subsequent 1-year-old fish, although no such correlation occurred between postlarval abundance and either the abundance of juveniles or the CPUE of subsequent 1-year-old fish. This indicates that significant mortality occurs throughout the larval stages and that year-class strength seems essentially determined by the juvenile stage. A strong year-class seems to occur when the boundary between stratified Hiuchi-nada, the spawning area, and vertically well mixed adjacent regions becomes obscure in June

Effect of an External Tag on Growth of Sabiefish (Anoplopoma fimbria), and Consequences to Mortality and Age at Maturity

Tagged juveniles from a strong year-class of sablefish (Anoplopoma fimbria) were sampled for 9 yr Tagged females were approximately 6 cm larger than untagged females at 50% maturity. Males showed little difference because of their slower growth rate. Untagged males and females matured one and 2 yr earlier than tagged males and females. From this same year-class length-at-age of untagged males and females was significantly larger than for tagged and recaptured fish. Untagged males and females were 6.5 and 8 cm larger than tagged fish by age seven. Smaller tagged fish were recaptured at lower rates than larger tagged fish, indicating that slower growth reduced recapture percentage, which we believe indicates a higher mortality rate. Because smaller size was associated with increased mortality and tagged fish grew slower than untagged fish, we concluded that tagged fish had higher rates of mortality than untagged fish. The increased age and size at 50% maturity and the increased juvenile mortality indicated the potential response of a population of sablefish and possibly other species to a reduction in individual fish growth. If our results apply to other species and other types of tags, investigators should be cautious in extrapolating from tagged to untagged populations.

Temperature-Dependent Growth of Juvenile Red King Crab (Paralithodes camtschatica) and its Effects on Size-at-Age and Subsequent Recruitment in the Eastern Bering Sea

A temperature-based growth relationship was derived for juvenile red king crab (Paralithodes camtschatica) from published data. Growth of annual cohorts of crab was simulated at various locations in Bristol Bay, AK, using long-term water temperature observations, and the resulting mean sizes were compared with those observed by the annual National Marine Fisheries Service eastern Bering Sea crab survey. Results indicate that mean age-at-recruitment is 3–5 yr greater than previously estimated, and has decreased from 9.5 to 7.5 yr over the past decade as a result of increasing sea temperature. Single year-classes require 3–5 yr for complete recruitment to the 'mature' size interval, and recruitment in any year may consist of crabs from four to five year-classes. The high landings and subsequent crash of the Bering Sea red king crab population during the late 1970's probably originated from one or more adjacent strong year-classes in the late 1960's which recruited over a period of several years, followed by a period of recruitment failure and high mortality. Future stock–recruitment studies should account for the effects of multi-year recruitment and year-class overlap.

Population Dynamics of Black Crappie in Lake Okeechobee, Florida, following Suspension of Commercial Harvest

Abstract Population dynamics of black crappies Pomoxis nigromaculatus in Lake Okeechobee, Florida, were monitored from 1981 to 1987 to document changes that occurred following suspension of commercial harvest for this species in 1981. Black crappie abundance increased substantially. Mean annual catch per unit effort (CPUE) of otter trawls increased from 1.9 fish/min in 1981 to 22.4 fish/min in 1987. In addition, the estimated winter-spring angling harvest of black crappies on the north end of the lake increased from 34,000 fish in 1980-1981 to over 700,000 fish in 1986-1987. Increased abundance of black crappies resulted from a reduction in annual mortality from 65 to 39% due to suspension of commercial harvest and recruitment of strong yearclasses spawned in 1980, 1983, 1984, and 1985. Multiple linear regression revealed that year-class strength of black crappies was positively related to water levels during the spring spawning season and to CPUE of January trawls for individuals at least 178 mm in lengt...

Patterns of Exploitation and Biological Status of Pollock (Pollachius virensL.) in the Scotian Shelf, Georges Bank and Gulf of Maine Area

Commercial exploitation of pollock in NAFO Divisions 4VWX and Subareas 5 and 6 has increased considerably over the past two decades as abundance of traditional groundfish species has declined. Total pollock landings from this area increased from less than 30,000 (metric) tons per year during the late-1960s to over 60,000 tons since 1985; USA recreational catches have contributed between 1,000 and 2,000 tons to the annual totals. Since 1977, when both the USA and Canada extended uni-Iateral jurisdiction over their fishery resources to 200 miles, domestic management programs for pollock have been either non-existent or ineffective in restricting total landings. Assessment and management advice for pollock has traditionally been predicated on the assumption of single unit stock within the region. Tagging studies and morphometric and meristic measurements, however, suggest possible separation of Gulf of Maine pollock from those on Browns Bank and Emerald Basin on the Scotian Shelf. The distribution of pollock eggs and larvae also indicates the presence of several spawning sites on the Scotian Shelf in addition to previously documented locations in the western Gulf of Maine. Sexual maturation and growth rates for pollock are similar throughout the region. Growth rates of males and females do not differ significantly, although the median size at maturity is slightly larger for males. The majority of pollock of both sexes become sexually mature during their third year. Recruitment has been consistent since the late-1960s with one or more relatively strong year-classes appearing throughout the region every 3-4 years. Pollock become fully recruited to the fishery between ages 6 and 7, although partial recruitment declines again after age 7. Estimates of total stock size (age 2+), derived from virtual population analysis (VPA), increased from 102 million fish in 1974 to 145 million in 1977, but declined to 97 million in 1980. Following recruitment of the 1979 year-class at age 2 in 1981, stock size increased to 168 million before declining to 113 million in 1988. Instantaneous fishing mortality (F) has exceeded Fo.i (0.29) but remained at or below Fm a, (0.57) throughout the 1970s. Fishing mortality has been close to Fm a however, in 4 of the 6 years since 1982. Analyses of research vessel survey data generally agree with results obtained from the VPA, indicating recent decline in stock abundance and biomass, and an increase in instantaneous total mortality. Equilibrium calculations suggest that fishing at Fa' would provide long-term yield of 53,600 tons from stock biomass of 317,700 tons, while fishing at Fm a, would provide yield of 58,100 tons from stock biomass of 204,600 tons. Corresponding spawning stock biomass levels at Fa' and Fm a, are 260,400 and 149,800 tons respectively. Although long-term yields are approximately 8% greater at the Fm a, level, fishing at Fa' provides for 55% increase in total stock and 74% increase in spawning stock biomass over those allowed under Fma, . 'Present address: Max Planck Institute, 1A Marconi Strasse, Julich D-5170, Federal Republic of Germany. 14 J. Northw. Atl. Fish. Sci., Vol. 9,1989

Factors Affecting Recruitment and Mortality Rates of Rainbow Smelt ( Osmerus Mordax) in Lake Erie, 1963–85

Recruitment of young-of-the-year (YOY)rainbow smelt ( Osmerus mordax) per parent was nqt dependent upon water temperature (rate of warming and average temperature) during spawning, Glugea infestation, or parental stock-size. Only in the eastern basin of Lake Erie was recruitment to age 1 + years-old positively correlated with the YOY index in the preceding summer. Recruitment of age 1 + year-old smelt was not related to parental stock-size. Recruitment (age 1 + years-old) per spawner was inversely related to the abundance of age 1 year-old smelt present during the year of hatch in both the eastern and central basins of Lake Erie. This could be taken as evidence to support the hypothesis that the alternating dominance of year-classes (1963–74) was caused by cannibalism of YOY by age 1 year-old smelt. After 1974, alternating year-class-dominance was not evident; we considered three possible explanations for the absence of strong year-classes. First, the size at age declined in both basins during the study so that more age classes could have fed optimally on young-of-year (YOY), resulting in a poorer survival of YOY smelt. Second, the harvest of smelt increased greatly after 1974, particularly for yearling smelt, so that year-class strength measured by the abundance of yearlings was reduced by fishing. Third, predation on smelt by stocked salmonids probably increased after 1974 as the stocking rates increased. Adult total mortality rates (ages 2 and 3 years-old) were positively related to stock density providing some evidence for density dependent mortality of smelt. There was no evidence that the variations in adult mortality were associated with the incidence of Glugea.