Spawner Abundance Research Articles

Maternal effects — a link between the past and the future

Variations in growth and condition of female fish change the viability pattern of the offspring. The influence of maternal effects on the reproductive potential, and hence recruitment, has gained support during the last decade, both from laboratory experiments and field investigations. Since such maternal effects are phenotypic and therefore the result of the age/size and condition composition of the spawning population, overfishing will cause the population to move away from its fitness peak. For this reason maternal effects are the only recruitment mechanism which can be directly managed through fishing regulations. This paper reviews the maternal effect of early larval survivorship in the field, in connection with the synchrony idea of J. Hjort and D. Cushing. The weak relation between spawning biomass and recruitment has discredited spawner abundance as a significant predictor of recruitment. Dynamic investigations on the maternal factors and the fecundity can be used to adjust estimates of spawning biomass. It is expected that this approach will clarify the significance of the spawning population in determining the level of recruitment.

ESTIMATION OF SALMON ESCAPEMENT: MODELS WITH ENTRY, MORTALITY AND STOCHASTICITY

Understanding the dynamics of Prince William Sound pink salmon requires knowledge of the size of the spawning population in a stream over time. Periodic aerial surveys provide observations on the number of spawners, but the lack of daily observations requires a model to fill in the gaps. We develop a differential equation framework to represent the dynamics of escapement during the season. An exponential population growth model with a time‐varying rate of growth is used for the number of spawners. The rate of growth consists of two primary components: the entry of salmon to the stream (escapement) and the mortality of spawners in the stream. The models for entry and mortality are also functions of time. The stochastic element of the model is based on a nonhomogeneous birth‐and‐death process which leads to a least squares estimation approach with either additive measurement or process errors. We illustrate the approach for a stream in Prince William Sound by fitting various models to observed spawner abundance, mortality counts from ground surveys and weir counts of the entry to the stream. We believe this approach could improve salmon escapement estimation, because the processes governing entry and mortality are explicitly considered.

Analysis of harvest strategies for red king crab, Paralithodes camtschaticus, in Bristol Bay, Alaska

A modifiable harvest rate constrained by a minimum spawning abundance (threshold) is currently used to set the annual harvest level for Bristol Bay red king crab, Paralithodes camtschaticus. A length-based simulation model was constructed to evaluate effects of recruitment, natural mortality, and handling mortality on this harvest strategy. Evaluation criteria included mean yield, stability of yield, harvest opportunity, and stability of spawning stock. Optimal mature male harvest rates were strongly negatively related to handling mortality. For any given harvest rate, handling mortality is a key factor influencing optimal thresholds. The current harvest strategy produces a high mean yield and low variability in yield under low handling mortality scenarios, but the population is at high risk of collapse with a high handling mortality. Given uncertainties of recruitment, natural mortality, and handling mortality estimates, we recommend reducing mature male harvest rate from 20 to 15% and maximum legal male harvest rate cap from 60 to 50%. If handling mortality rate is greater than 30%, then we recommend increasing the threshold from 6600 to 11<|>000 metric tons of effective spawning biomass. Our recommended harvest strategy produces a mean yield similar to the current harvest strategy and safeguards against recruitment overfishing.

Comment and reanalysis: paradigms for recruitment studies

“Is recruitment related to spawner abundance?” was addressed by Myers and Barrowman (1996) using the largest data set ever compiled in fisheries (364 time-series). We posed three questions: (i) does the highest recruitment occur when spawner abundance is high? (ii) does the lowest recruitment occur when spawner abundance is low? and (iii) is the mean recruitment higher if spawner abundance is above the median rather than below? We found that when there is a sufficient range in spawner abundance the answer to all three questions is almost always yes. An important aspect of the analysis, and a point completely ignored by Gilbert (1997), is that not all spawner recruit time-series have the same information content. This point is essential in any meta-analysis and is one of several reasons why I cannot accept Gilbert’s analysis. Gilbert (1997) claims, using an earlier version of my data compilation and a method that is totally untested, that the conclusions of Myers and Barrowman (1996) are wrong. He implies that eggs are not in fact required to produce baby fish, and that there is absolutely no need to conserve spawners to ensure future recruits. This is Gilbert’s “new paradigm”; however, it is the most commonly used paradigm in what sometimes passes for fisheries management. The failure to recognize the need to conserve spawning biomass is a principal reason for the disasterous collapse of the formerly great cod fisheries in Eastern Canada (Hutchings and Myers 1994; Myers et al. 1996, 1997). These fisheries were managed using Gilbert’s so-called new paradigm, i.e., the assumption that recruitment was independent of spawner abundance. Those wishing to follow Gilbert’s advice have many precedents of failure to warn them of their folly. However, I believe that his advice is very popular because it gives an excuse to overexploit now with no concern for future recruitment (again there is hardly anything new about this idea; Huxley 1884). If Gilbert is correct, there should be lots of new recruits in these six cod populations that collapsed in Eastern Canada in spite of the fact of very low spawner abundance. Unfortunately, this is not the case.

Population Dynamics of Exploited Fish Stocks at Low Population Levels

Models of population dynamics in which per capita reproductive success declines at low population levels (variously known as depensation, the Allee effect, and inverse density-dependence) predict that populations can have multiple equilibria and may suddenly shift from one equilibrium to another. If such depensatory dynamics exist, reduced mortality may be insufficient to allow recovery of a population after abundance has been severely reduced by harvesting. Estimates of spawner abundance and number of surviving progeny for 128 fish stocks indicated only 3 stocks with significant depensation. Estimates of the statistical power of the tests strengthen the conclusion that depensatory dynamics are not apparent for fish populations at the levels studied.

Seasonal, spatial and interannual variability in the reproductive dynamics of the grooved tiger prawn Penaeus semisulcatus in Albatross Bay, Gulf of Carpentaria, Australia: the concept of effective spawning

An index of population fecundity based on the abundance and fecundity of spawners was used to describe the spawning patterns of a population of Penaeus semisulcatus de Haan in the Albatross Bay region of the Gulf of Carpentaria, Australia. Between March 1986 and March 1992, 66 monthly sampling cruises were completed and a total of 133238 prawns were caught in 1986 trawls. Subadult females from the spring (August–November) spawning recruit to the offshore population in summer (December–January). Some of these mature and spawn in late-summerautumn (January–March) at ∼6 mo old, but most spawn in spring (August–November) at ∼12 mo old. The subadult females from the late-summer-autumn spawning recruit to the population in May–July, and some spawn in spring (August–November) at ∼6 mo old. As a result there is a mixture of 6 and 12-mo-old spawners in spring. The effective spawning periods for stock renewal, based on subsequent larval, postlarval and juvenile abundances, and recruits, are spring (August to November) and late summer (January–February). In 2 of the 6 yr, the spring and late-summerautumn spawning patterns were clearly bimodal; in the other years the late-summer-autumn peak was less distinct, although spawning continued over this whole period. Both the absolute and relative sizes of the seasonal peaks varied between years. Population fecundity was found to be a function of the complex interactions of the abundance and size structure of the females and the proportion of spawners. P. semisulcatus moves offshore and disperses between February and May. By May, most of the adults that recruited the previous summer are located offshore in depths >40 m. After July, they move to shallower waters (<40 m). In September–October they spawn, together with the component of autumn recruits, in depths of 20 to 35 m. The variation in seasonal, spatial and size composition patterns of the spawners provides the basis for description of the subset of the overall spawning stock (“effective spawners”) that contributes to subsequent recruitment.

Recruitment variation in abalone: Its importance to fisheries management

Recruitment failure has been implicated in the decline of several abalone fisheries. Traditionally, fisheries scientists invoke theoretical stock-recruitment relationships to predict trends in abundance of an exploited stock under various harvest regimes. The empirical evidence in support of a positive relationship between spawning stock and recruits is not strong. A further problem in interpretation of such relationships is that both 'stock' and 'recruitment' have various definitions in fisheries and ecological literature. The definition of a stock for abalone is not clear. As emphasized in this review, which considers each stage in the life history of abalone, the abundance of spawners is one of many sources of variation in recruitment. The evidence for invertebrates, particularly those with high fecundity, is that recruitment varies independently of the abundance of spawners. This is also the case for abalone, where recruits have been measured as the density of immediate post-settlement individuals, juveniles, or as adults entering the exploitable stock. A problem with stock-recruitment hypotheses is that they have intuitive appeal. It is considered 'dangerous' to manage fisheries under the assumption that a reduction in the number of spawners by fishing will not affect recruitment. Such danger to abalone stocks has been more recently assessed by egg-per-recruit analyses, whereby various harvest strategies are examined relative to reference points for egg production. These studies are reviewed and assessed relative to the often conflicting aims of managers and scientists. This review of studies of recruitment variation in abalone emphasizes the need for a more rigorous, autecological approach to stock assessment in which field experiments are conducted over realistic spatial and temporal scales, permitting robust testing of hypotheses.

An Evaluation of Lake Trout Reproductive Habitat on Clay Banks Reef, Northwestern Lake Michigan

The extinction of the native populations of lake trout (Salvelinus namaycush) in Lake Michigan in about 1956 has been followed by a decades-long attempt to reestablish self-sustaining populations of this valuable species in habitats it formerly occupied throughout the lake. One of the most recent management strategies designed to facilitate recovery was to make a primary management objective the establishment of sanctuaries where stocked lake trout could be protected and self-sustaining populations reestablished. In the present study we employed habitat survey and mapping techniques, field and laboratory bioassays, egg traps, sediment traps, and gill nets to examine the potential for successful natural reproduction by stocked lake trout on Clay Banks Reef in the Door-Kewaunee sanctuary in Wisconsin waters of Lake Michigan. Our study revealed (1) there was suitable habitat on the reef to support the production of viable fry, (2) spawner abundance on the reef was the highest recorded in the Great Lakes, and (3) eggs taken from spawners on the reef and held on the reef in Plexiglas incubators hatched and produced fry that survived through swim-up. We conclude that Clay Banks Reef has the potential to support successful natural reproduction by stocked lake trout.

Lake Trout Restoration in the Great Lakes: Stock-Size Criteria for Natural Reproduction

We examined the question of whether the lake trout restoration program in the Great Lakes has developed brood stocks of adequate size to sustain natural reproduction. Stock size criteria were developed from areas of the Great Lakes where natural reproduction has been successful (defined as detection of age-1 or older recruits by assessment fishing). We contrasted them with stocks in areas with no natural reproduction. Based on the relative abundance of spawners measured in the fall and the presence or absence of natural reproduction in 24 areas of the Great Lakes, we found three distinct sets of lake trout populations. In seven areas of successful natural reproduction, the catch-per-unit-effort (CPE) of spawners ranged from 17 to 135 fish/305 m of gillnet. Stock sizes in these areas were used as a gauge against which stocks in other areas were contrasted. We conclude that stock densities of 17–135 fish/305 m of gill net are adequate for natural reproduction, provided that all other requirements are met. No natural reproduction has been detected in seven other areas, where CPEs of spawners ranged from only 3 to 5 fish/305 m. We conclude that spawning stocks of only 3–5 fish/305 m of net are inadequate to develop measurable natural reproduction. Natural reproduction has also not been detected in ten areas where CPEs of spawners ranged from 43 to 195 fish/305 m of net. We conclude that spawning stocks in these ten areas were adequate to sustain natural reproduction, but that some factor other than parental stock size prevented recruitment of wild lake trout.

A Model of an Internally Supplemented Population

Abstract A model of the combined abundance and stock productivity of wild and hatchery spawners was developed as a tool to analyze the use of an internal form of stock supplementation to rebuild depressed populations. The model incorporated five key variables: Abundance and productivity of the wild spawners, abundance and productivity of the hatchery spawners, and the proportion of hatchery spawners. The model assumes discrete generations, a hatchery stock that is taken solely from the wild stock, and no genetic change during hatchery culture. Quantitative analyses delineated the existence of specific combinations of variables that will result in rebuilding the abundance and maintaining the genetic diversity of depressed populations. The model can be used as a tool for screening proposed supplementation projects and evaluating the performance of ongoing projects.

Spotted Seatrout Distribution in Four Coastal Louisiana Estuaries

We analyzed experimental gill-net catches of spotted seatrout Cynoscion nebulosus at 28 stations in various estuarine habitats over 3 years (1988–1990). Cluster analysis indicated that the 28 stations reduced to three “natural” groupings representing salinity zones that correspond to upper (oligohaline, 0–9‰), intermediate (mesohaline, 10–14‰), and lower (polyhaline, 15–30‰) estuarine zones. Principal components analysis applied to log-transformed gill-net catches yielded a two-factor model that explained 78% of the variation in the data; factors were interpreted as recruit size (females smaller than 31 cm in total length and males smaller than 26 cm) and spawner size (females larger than 31 cm and males larger than 26 cm). Both recruit and spawner models were highly significant (P < 0.0001), and the zone-by-season interaction accounted for the greatest proportion of the variation in the models (P < 0.0001). Abundances of recruit and spawner spotted seatrout were greatest in the lower estuarine zone (15–30‰) during the spawning season (May–August), when both recruit and spawner abundances had high positive correlations with salinity (P < 0.01). After the spawning season (September–December), spawners were uniformly distributed over all estuarine zones, whereas recruits were significantly more abundant (P < 0.01) in the upper estuarine zone (0–9‰). During this time, recruit spotted seatrout abundance showed a high negative correlation with salinity (P < 0.01).

Abundance, Size, and Recruitment of Pink Salmon ( Oncorhynchus Gorbuscha) in Selected Michigan Tributaries of the Upper Great Lakes, 1984–1988

The abundance and size of pink salmon spawners and fry were studied in selected Michigan tributaries of Lakes Huron, Michigan, and Superior from 1984 to 1988. Pink salmon populations in Lake Superior have declined since the early 1980s. In Lake Huron, populations began increasing in 1983 and have leveled off at relatively high numbers. Lake Michigan populations have increased. Male spawners were significantly longer than females in all lakes. Lake Michigan fish were significantly longer than Lake Huron and Lake Superior fish, but no significant difference was observed between the latter two groups. Male spawners averaged 482 mm (1,045 g) in Lake Michigan, 418 mm (574 g) in Lake Huron, and 410 mm (567 g) in Lake Superior. Females averaged 449 mm (840 g) in Lake Michigan, 389 mm (450 g) in Lake Huron, and 388 mm (466 g) in Lake Superior. No significant length differences were observed in outmigrating fry from tributaries of the three lakes. Fry from individual tributaries averaged 31.94 mm (0.138 g) in Lake Huron, 32.64 mm (0.132 g) in Lake Michigan, and 31.96 mm (0.117 g) in Lake Superior. Fry recruitment was variable among years and dependent upon spawner abundance and over-winter flow conditions. Survival estimates from egg deposition to fry outmigration were poor (0.15% to 0.45%), which demonstrates the importance of riverine environments to Great Lakes pink salmon abundance.

Recovery of an Offshore Lake Trout ( Salvelinus Namaycush) Population in Eastern Lake Superior

The lake trout ( Salvelinus namaycush) population at Stannard Rock, Michigan, an isolated offshore reef in eastern Lake Superior, was monitored each spring from 1959–79 using a permit assessment gill net fishery. This population, like nearly all of those in inshore waters, declined to low levels during the years of intense predation by the sea lamprey ( Petromyzon marinus) in the late 1950s and early 1960s. After sea lamprey control began in 1961, the abundance of native lake trout at Stannard Rock began to increase slowly in the 1960s and was limited to recruitment into the fishable stock of native fish that were juveniles during the years of high sea lamprey activity. By the early 1970s, lake trout abundance increased sharply and remained at a high level. This rapid recovery resulted from several strong year classes produced by the small spawning aggregations that reached maturity in the 1960s. Reproduction occurred in all years during and after the peak of sea lamprey activity in 1959. The strengths of year classes produced in 1963–1968 were related to the relative abundance of spawners the previous fall.

Detection of Environmental Influence on Recruitment Using Abundance Data

Environmental influences on recruitment are usually detected by correlation analysis using environmental and recruitment data. When recruitment data are unavailable, abundance data are sometimes used instead. An abundance time series can be described as a convolution of the recruitment time series, and consequently, when it is used as a proxy for recruitment, the magnitude of correlations is lower and the probability of both type I and type II errors is greater. Two approaches can restore the correlation between the environmental series and the recruitment series: (1) convolution of the environmental series and (2) deconvolution of the abundance series. Convolution of the environmental series increases the variance of computed correlation coefficients and can further increase the probability of type II errors. Deconvolution of the abundance series can recover the correlation between the environment and recruitment without increasing the variance of sample correlation coefficients or the probability of type II errors, as long as the deconvolution is stable and errors in the data are relatively small. When errors in the data are sufficiently large or the deconvolution is sufficiently unstable, the best results may be obtained by simply using abundance as a recruitment index. We evaluate application of the deconvolution technique to chinook salmon (Oncorhynchus tshawytscha) catch and spawner abundance series using both real and simulated data.

Acoustic study on the abundance and distribution of anchovy spawners and recruits in South African waters

Direct surveys of Cape anchovy Engraulis capensis spawner and recruit biomass in South African coastal waters were made with a 38-kHz calibrated echo-sounder and custom-built digital echo-integrator. The spawning surveys were conducted each November between 1983 and 1985, predominantly on the Agulhas Bank. Recruit surveys were conducted in autumn 1985 and 1986, mainly on the West Coast. The spawning fish were widely distributed across the continental shelf, whereas recruit concentrations were highest close inshore. Employing a target strength per kilogramme expression for herring Clupea harengus, the spawning biomass in 1984 was estimated at 1,06 million tons and that in 1985 at 0,98 million tons. Similarly, recruit biomass in May/June 1985 and June 1986 was estimated at 310 and 466 thousand tons respectively. From an analysis of error in the latter two estimates, it was considered approximately 80 per cent certain that the recruitment in 1986 was greater than in 1985. The estimates of spawning biomass are 3–4 times higher than earlier estimates made by Virtual Population Analysis (VPA) of commercial catch data, and suggest that the stock is less seriously threatened by present catch levels than was indicated by VPA. It is concluded from the distribution and age structure of the spawning population that the VPA estimates are invalid because of the failure of commercial vessels to sample the major part of the adult population.

Population Characteristics of Adult Pink Salmon in Two Minnesota Tributaries to Lake Superior

Pink salmon, Oncorhynchus gorbuscha, have adapted well to the Lake Superior environment and have become a significant member of the Great Lakes salmonid community. However, to properly manage pink salmon in this new environment, an improved understanding of their population characteristics is necessary. To gather population information, the following parameters were estimated for the 1981 and 1982 runs of pink salmon in the Cascade and Cross rivers, Minnesota: 1) absolute abundance of spawners, 2) age, size, and sex distribution of the spawners, 3) fecundity and growth of returning adults, and 4) egg deposition, egg fertilization rates, and embryo development. Trap nets were used to collect upstream migrant pink salmon, while electrofishing, carcass counts, and creel censuses were used to obtain recaptures. Left pectoral fin rays were used for aging and growth back-calculation. Redd samples were collected to estimate egg fertilization rates and embryo development. It was found that Lake Superior pink salmon are similar to their Pacific Coast counterparts, except that they are smaller and 3-year-old fish are common. The latter may preclude the genetic divergence based on spawning stock which is apparent in the Pacific. The sex distribution of 3-year-old pink salmon was highly skewed towards females. Three-year-old female pink salmon had a lower fecundity and a poorer egg quality than 2-year-old females. The major factor controlling the abundance of Lake Superior pink salmon appears to be limited spawning habitat.

Early Life History of Redfish (Sebastes spp.) on Flemish Cap

Studies were carried out on Flemish Cap, 1978–82, to assess fish spawning cycles, the distribution, abundance, and growth of early life stages, and their relationship to environmental factors. Redfish larvae (Sebastes spp.) were the most abundant fish larvae found on Flemish Cap. Redfish began releasing larvae during March, reaching an abrupt peak in late April. Larval abundances of 733 larvae∙m2 were observed during the last week of April 1979, with highest sampled larval abundance for the study area being 6.8 × 1012 larvae. Redfish larvae first appeared in the southwest corner of Flemish Cap and within 3 wk were found in waters throughout the area over depths &gt; 200 m. In July the survivors were concentrated over the Cap supporting the concept that Flemish Cap redfish constitute a distinct group. Larval growth for redfish was exponential through the periods sampled. However, in 1981, growth rate ranged from 0.40 to 1.66%∙d1. While larval growth was significantly correlated with average surface water temperature, high temperatures appeared to reduce larval growth. Slow growth during warm years appeared to be related to increased larval mortalities.

Reproductive Strategy of Dwarf and Normal Arctic Charr (Salvelinus alpinus) from Vangsvatnet Lake, Western Norway

In this paper, we investigate the reproductive strategy maintaining dwarf and normal phenotypes of an Arctic charr (Salvelinus alpinus) population. Both morphs occurred together in the spawning area, but the peak abundance of dwarf spawners was significantly deeper, and later in the spawning period, than normal spawners. Both phenotypes attained sexual maturity at the same age and did not differ in length-specific fecundity or egg diameter. Our data support the hypothesis that the females matured sexually at the age that maximizes their fitness within the constraints imposed by growth and survival. The two female morphs appeared not to have equal fitness; these differences may be due to random events. The males were more heterogeneous by size than the females when maturing sexually. This may be due to intrasexual competition between males making large males the principal spawners. The dwarf males may circumvent the effects of intrasexual competition by mimicking immature fish, sneak at the redds of normal females, and participate in the spawning acts of normal charr pairs. Also, they may fertilize the eggs of dwarf females spawning in deeper areas.The two charr morphs developed from a uniform young phenotype (parr). The dwarf spawners were precocious individuals maturing sexually in their parr stage, whereas normal spawners had a color shift, analogous to smoltification in anadromous charr, before they attained sexual maturity. The different spawning coloration of the two morphs may be due to sexual selection.Key words: spawning, growth, fecundity, mortality, sexual selection, maturing strategy

Two-Year Cycles of Abundance of Age-0 Rainbow Trout in Lake Superior Tributaries

Abstract Two-year cycles of abundance of age-0 rainbow trout (Salmo gairdneri) occurred in two of five Michigan streams where annual population estimates were made in 1967–1974. The cause(s) was not determined, but the cycles could not be related to density of juvenile salmonids, abundance of spawners, meteorological conditions, or abiotic stream characteristics.

Variations in Fecundity of White Crappie in Conowingo Pond, Pennsylvania

Annual variations in fecundity of the white crappie (Pomoxis annularis) occurred in Conowingo Pond, a 35.8-km/sup 2/ impoundment on the lower Susquehanna River, Pennsylvania. The relative fecundity of 119 white crappies in 1971 to 1973 ranged from 177 to 295 eggs per gram of fish. In 1974 to 1976 the relative fecundity of 154 white crappies ranged from 259 to 304. Although length, age, and weight of the fish were all significantly correlated with fecundity, weight explained most variation and thus was the best predictor. As determined by analysis of covariance, the relative and mean adjusted fecundity were significantly different (P < 0.001) among years. Random variations in egg number were greater among years than between preoperational (1971 to 1974) and postoperational (1975 to 1976) periods with respect to start-up of a nuclear power station. Some evidence of density-dependent variation in fecundity existed in the white crappie population; fecundity increased at low white crappie density. However, annual variations in fecundity were not significantly correlated with variations either in average physicochemical conditions or in the abundance of spawners.