Robin Mahon

Accuracy of catch-effort methods for estimating fish density and biomass in streams

SynopsisAt each of 11 localities a section of stream was closed off with nets and an electrofisher used to estimate the abundance of fishes in the section. Each section was fished from 5–7 times with each fishing equalling one unit of effort. Using the catch-effort methods of Leslie, DeLury and Ricker, separate estimates were made for each species. In several cases species were split into size groups and estimates made for each group. The fish remaining in each section after the fishings were collected using rotenone. Thus the estimates could be compared to the actual number of fish present. Estimates were considered to be either ‘good’, if the regressions used in the above methods were statistically significant or ‘bad’ if they were not significant. Lower limits for the number of fish and mean weight of a fish for ‘good’ estimates were identified.The Leslie and Ricker estimates, which did not differ significantly, were least in error. They tended to underestimate (−21.6% on the average for the Leslie method). Direct estimates of biomass did not differ significantly from those made using the estimates for numbers and the mean weight of fish caught. The interrelationships among variables such as mean weight, numbers, catchability, density, biomass, number of catches used, proportion of fish taken during the estimate, number of fish in the last catch and their relationships with the error of the estimates were examined using correlation and principal components analysis. Error was most closely related to the proportion of fish collected. The effects of other variables such as mean weight affected error through catchability and subsequently the proportion of fishes caught. It was not possible to predict a significant proportion of the error using variables which could be measured without a complete collection. The effects of locality, electrofisher, and species on error were examined. Each accounted for a significant proportion of the variability in error but primarily by affecting the proportion of fish caught.These results suggest that the most appropriate way of decreasing error would be to increase the total effort and consequently the proportion of fish collected. This would be best done by increasing the number of fishings used in the estimate.Catchability tended to decrease in successive fishings. The observed trends in changing catchability accounted for most of the error. Size-selectivity, which was evident as a change in mean weight in successive catches, was not significantly associated with changing catchability.

Distribution, community structure and production of fishes in the upper Speed River, Ontario: a preimpoundment study

SynopsisThe upper Speed River, on which a dam was under construction, was surveyed through fourteen qualitative and six quantitative collections to determine the distribution of fishes in the river above Guelph, Ontario and in a tributary, Luteral Creek. At the quantitative localities the removal method was used to estimate density and standing crop of all species. At three of these localities age and growth of the dominant species were determined and total ecological production calculated.Two approaches to the estimation of production for the 0–1 age class were compared and found to result in differences of 33–39% in the estimates of total production, thus illustrating the extent to which methodology may affect such estimatesThe dominant species in the river were, Semotilus atromaculatus, Notropis cornutus, Pimephales notatus, Rhinichthys atratulus, Hypentelium nigricans, Catostomus commersoni, Ambloplites rupestris, Micropterus dolomieui and Etheostoma flabellare. Density ranged from 11126 to 74765 individuals per hectare and standing crop from 32.4 to 190.0 kg ha−1. Production values at the three localities were 15, 19 and 54 kg ha−1 yr−1 , and are low compared with estimates for other fluvial systems. Comparison with a 1951 survey of the Speed River revealed considerable changes in the composition and distribution of fish species. The ranges of several cold-water species had contracted towards the headwaters, whereas several warm-water species had extended their ranges up the river

Structure and resilience of a tidepool fish assemblage at Barbados

Fish collections from 19 tidepools on a rock plateau at Martins Bay, on the east coast of Barbados, taken on three occasions (1981,1983 and 1987) contained 2078 individuals of 63 species. The number of species, individuals and total biomass increased with pool size. Partial residents, primarily juveniles of reef species, comprised 44% of species, 36% of numbers, and 26% of biomass. True and partial residents were of similar sizes. Most of the latter grow to larger sizes than those observed in the pools, indicating that the use of tidepools by fishes is size-dependent. Species richness, numbers of individuals and biomass in individual pools was positively associated with pool size. These relationships did not vary among sampling occasions. Species composition and relative abundance was also found to be similar among sampling occasions, leading to the conclusion that the tidepool assemblages are resilient and stable.

Hybridization of Cyprinus carpio and Carassius auratus, the first two exotic species in the lower Laurentian Great Lakes

SynopsisA preliminary comparison of Cyprinus carpio × Carassius auratus hybrids with the parent species is made by means of hybrid indices for 29 morphometric and 19 meristic characters. Of these 48 characters 29 were intermediate, seven were identical with one of the parents and 12 were extreme. Marked osteological differences in the crania of the parent species were noted as possible diagnostic characters for the identification of hybrids.

Ecological fish production in Long Pond, a lakeshore lagoon on Long Point, Lake Erie

SynopsisLong Pond, a recently formed 2.4 ha lagoon on Long Point, Lake Erie (Canada), was treated with piscicide and found to contain 47,768 fishes of 22 species. Nine dominant species (Notropis heterolepis, Pimephales notatus, Notemigonus crysoleucas, Lepomis gibbosus, L. macrochirus, Perca flavescens, Cyprinus carpio, Micropeterus salmoides and Carassius auratus) comprised 91% and 97% of total numbers and standing crop respectively. Age, growth and production were determined giving a total fish production estimate (corrected for all species) of 87.5 kg ha−1 y−1 for this lagoon. Dominant species were deemed either invaders (Cyprinus carpio, Carassius auratus), which had entered the lagoon to spawn and been trapped, or residents. The invaders comprised 82% of the 277.8 kg ha−1 total standing crop. Their successful spawning and subsequent escape of the young from the lagoon would have resulted in some 615 kg y−1 of fish production outside the lagoon. Resident stocks were dominated by young fishes, a characteristic of exploitation or early colonization. This may have been due to the recent formation and subsequent expansion of the lagoon or high annual mortality due to extreme physical conditions but was most likely the result of excessive predation by Micropterus salmoides. Relationship between biomass and production show that the chance inclusion of the invaders produces an immediate shift from a characteristically early to a characteristically late phase of ecological development.

Fish community structure in lakeshore lagoons on long point, lake Erie, Canada

SynopsisThe interaction of fluctuating water level and the Long Point topography is discussed with regard to the formation of lake-shore lagoons and their subsequent invasion by fishes. The species composition of fish communities in these lagoons was determined by 12 collections from lagoons (23 m2 to 25 ha) which yielded 33 fish species. The relationship of the number of fish species to lagoon area was lognormal; that is, with increasing lagoon area the number of species rose rapidly at first but soon decreased sharply, probably limited by the number of species capable of surviving in the habitat. The marked effect of area on species number in small lagoons is considered in the context of a hypothetical seasonal pattern of occasional invasion and continuous extinction.A sequence of reproductive guilds was found. In beach lagoons psammophils and lithophils were dominant. In a large vegetated lagoon in the interior phytophils and speleophils were dominant. This sequence was paralleled by an increase in the proportion of guarding and nesting species in the communities. The sequence was disrupted in the small interior lagoons where adaptations for survival were more important than reproductive strategy.

Seasonal abundance and reproduction of the fourwing flyingfish, Hirundichthys affinis , off Barbados

The seasonal abundance and reproductive cycle of the flyingfish Hirundichthys affinis was studied near Barbados. H. affinis shows large seasonal variation in abundance. They are common between December and June and scarce from July to November. The population size structure of H. affinis was determined by fishing with four different mesh sizes throughout one year. There appear to be two cohorts of H. affinis present in May and June, one of immature fish with a mean size of 19.8 cm FL, and one of mature fish with a mean size of 21.7 cm FL. From September to April there is only a single cohort of maturing fish with a mean size that increases from 20.4 cm FL in September to 22.0 cm FL in April. H. affinis spawns throughout the fishing-season (December–June) and shows a peak in spawning activity from March to June. Immature fish are present in Barbados waters from July to November but in very low abundance. Surface water temperature, wind speed and swell height are the only environmental factors with which catch rate of H. affinis showed significant correlation. Catch rate appears to be negatively correlated with temperature, whereas for wind speed and swell height there are optima. Three hypotheses for the seasonal variation in abundance of H. affinis were examined. The hypothesis that this species remains dispersed near Barbados during the off-season was rejected. The other hypotheses were refined. Both post-spawning mortality, resulting in an interval of low abundance between cohorts (non-overlapping generations), and migration of fish away from Barbados at the end of the fishing season, remain possible explanations for the observed seasonal variation in abundance of this species.

Selectivity of Experimental Gillnets for Fourwing Flyingfish, Hirundichthys affinis, off Barbados

Four experimental gillnets of stretched mesh sizes 2.54, 3.18, 3.81 and 4.45 cm (1.00, 1.25, 1.50, 1.75 inches) were fished together to allow estimation of gillnet selectivity for Hirundichthys affinis, by the method of comparing the catches of fish in the various length classes in the different nets. Only the 3.18 cm and 3.81 cm nets caught appreciable numbers of fish. The combined selectivity of the 3.18 cm and 3.81 cm nets is close to unity for the range of sizes of H. affinis which are available to the nets. Therefore, the size distribution of the combined catch for these nets can be used as an estimate of the size distribution of the available stock. The length frequency of fish caught in the 3.81 cm net coincided closely with the selectivity curve, indicating that this net effectively targets the sizes of fish available off Barbados. An increase or decrease in net mesh size by 0.64 cm (0.25″) would be expected to result in a substantial reduction in catch per unit effort.

A Comparison of Spawning, Harvested, and Die-Off Rainbow Smelt, Osmerus Mordax, in Eastern Lake Erie

Rainbow smelt were collected in the area of Long Point, Lake Erie from 1) a commercial trawler, 2) spawning runs in streams, and 3) die-offs along beaches. The parasite Glugea hertwigi was found to have no effects on the fecundity or condition of smelt. However, infested smelt grew more slowly than uninfested ones. The percentage of infested smelt in die-offs was not significantly higher than in the spawning stock, suggesting that the parasite was not the primary cause of the mortality. Growth, maximum age, and fecundity of smelt in the samples were lower than in most other smelt populations for which data are available. Since there are no data available for Lake Erie smelt prior to or during the development of the fishery, it is not possible to assess if the observed population structure is the result of exploitation. Sex ratio changes suggest that spawning smelt may remix with the harvested stock gradually throughout the summer.

A second look at bluegill production in Wyland Lake, Indiana

SynopsisAn error in the 1962 estimate of bluegill production for Wyland Lake, Indiana, is considered. An approximate corrected value of 625 kg ha−1 y−1 is estimated from the original data. This value is approximately six times the original estimate.