Spatial distribution, biomass and population dynamics of Mysis relicta in Lake Michigan

The abundance, biomass, and life history traits of Mysis relicta were evaluated in the spring, summer, and autumn of 2000 at 8 nearshore (45 m) and 8 offshore (75–110 m) stations throughout central and southern Lake Michigan. Abundance was also evaluated on a smaller scale during June 2000 at 4 nearshore and 4 offshore stations in southeastern Lake Michigan. For large-scale sampling, the abundance of M. relicta did not differ among locations in the spring. In the summer and autumn the abundance of M. relicta was similar among offshore stations with the exception of one station each season; for nearshore stations, abundance was generally highest off Pentwater, Michigan. The abundance of mysids was not consistently high for central or southern basin sites, although overall biomass was higher in the southern basin each season. Abundance of Mysis was positively correlated with bottom depth, but not with bottom water temperature, surface water temperature, or mean chlorophyll concentration. Within the smaller region in southeast Lake Michigan, the abundance of M. relicta differed among locations for both nearshore and offshore stations. Brood size and size of reproductive females did not differ among lake wide locations, but the proportion of females with broods and the size distribution of M. relicta did.     

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0⁺ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.     

Distribution and breeding biology of offshore pelagic cyprinids and catfish in Lake Malawi/Niassa

Synopsis Lake Malawi/Niassa is a permanently stratified lake, consisting of an oxygenated layer of 230 m depth that overlies an anoxic zone that extends to the lake bottom at a maximum depth of over 700 m. Some fish are found throughout the oxygenated zone of the offshore waters, and lead a pelagic existence, although many of the species form part of the demersal community in shallower waters where the lake-bed is oxygenated. This paper reports on the distribution of cyprinids and catfish at six offshore locations, sampled with a mid-water trawl and gillnets, along the north-south axis of the lake. These two groups formed, respectively, 5% and 13% of the offshore fish biomass in the trawl catches. The offshore cyprinids comprised the small lake-spawningEngraulicypris sardella, the only species to have pelagic larvae, and the larger river-spawningOpsaridium microcephalum andO. microlepis. These cyprinids occurred mainly in the upper 100 m of the water column. The biomass of both larvae and adults ofE. sardella showed considerable fluctuations over the two years of study. The spawning site ofE. sardella remains a mystery, but is no longer believed to be in the offshore pelagic zone. The most abundant catfish was the mochokidSynodontis njassae, that exhibited diurnal movements from just above the oxic-anoxic boundary layer during the day to the surface layers at night. The clariid catfish of the genusBathyclarias were caught infrequently offshore, but owing to their large size, they could form a significant part of the biomass. Their abundance was difficult to quantify, as it was believed that the majority are active in the near-surface layers and were under-sampled by the trawl and gillnets. Information on some aspects of the breeding biology is included in the paper, although our knowledge in this area is very incomplete.     

The spatial patchiness of Pacific herring larvae

Synopsis The spatial patchiness at age was measured for Pacific herring,Clupea harengus pallasi, larvae of a small coastal inlet. Lloyd's index of patchiness decreased from 3.8 at hatching to a minimum of 1.5 at 25–30 d and then increased to 2.5 at 50 d. This two-cycle pattern resembles that measured by others for the larvae of other species of schooling pelagic fishes. It differs by being several times lower in magnitude. This is attributed to two factors: (1) a rapid reduction in patchiness from a relatively high level in the demersal egg interval to a lower level in the planktonic larval interval as larvae are released into the water column over a period of 2–4 days, and (2) foraging by larval herring for prey that are larger, more dilute, and less patchy than the prey of other, smaller pelagic fish larvae.     

To produce many small or few large eggs: a size-independent reproductive tactic of fish

Summary We demonstrate here the existence of a range of size-independent reproductive tactics in teleostean fish involving the allocation of a size-dependent reproductive effort between fecundity and egg size. Despite considerable evidence that larger eggs and the larvae hatching from them are more likely to survive than smaller ones, we found no evidence of evolutionary trends towards greater egg sizes. Fish with pelagic eggs tend to spawn many, and therefore small, eggs, whereas demersal spawners tend to produce large, and therefore few, eggs. Maximizing egg number should increase the number of eggs hatching in suitable locations in the patchy pelagic environment and, hence, increase the reproductive success of pelagic spawners. On the other hand, the reproductive success of demersal spawners, which reduce the variance in growing conditions experienced by the off spring, should be more dependent on the survival of the individual larvae, which increases as egg size increases.     

利氏才女虫幼体发育的研究

对利氏才女虫Polydora ligni幼体的发育过程及各发育阶段形态结构特点作了观察研究,结果表明在温度(16±3)℃下,卵在卵袋中经过4 d发育成3刚节浮游幼体后进入水体开始浮游生活,13 d后14刚节的浮游幼体开始变态,19 d发育成21刚节后开始沉落转为底栖生活,21 d后23刚节的底栖幼体完成变态营管栖生活.     

Comparison of sleds versus plankton nets for sampling fish larvae and eggs

Fish larvae and fish eggs were sampled from the inshore waters of eastern Lake Michigan from 1978 through 1980, using a benthic sled and a plankton net towed within 0.5 m of the lake bottom. Differences between estimates of ichthyoplankton abundance based on the benthic sled and those based on the plankton net towed near bottom were examined along with interactions between gear, bottom depth, and time of day. Time of day was determined to be an important factor in comparing these two gear, but data were inconclusive as to the effect of depth on gear differences. Abundance of fish eggs calculated using sled tow data was significantly higher than that for the plankton net. For nighttime collections, density of alewife Alosa pseudoharengus larvae sampled in the plankton net significantly exceeded that for the sled, whereas density of spottail shiner Notropis hudsonius larvae based on sled data was significantly higher than that based on the plankton net for day sampling. Overall, the plankton net appeared to be adequate for sampling abundance of alewife larvae, while the sled was preferred for sampling fish eggs, spottail shiner larvae, and the following less common, but apparently demersal larvae: trout-perch Percopsis omiscomaycus, johnny darter Etheostoma nigrum, ninespine stickleback Pungitius pungitius, and slimy sculpin Cottus cognatus.     

Year-to-year variation in larval fish assemblages of the Southern North Sea

In order to test the temporal stability within and the reproducibility of larval fish assemblages between years, the larval fish assemblage at Helgoland Roads, North Sea (NE Atlantic) was quantitatively sampled almost daily from January 2003 to December 2005. The survey resulted in a total of 462 samples containing 50,632 larval fish of at least 42 taxa. In winter the larval fish assemblage was mainly dominated by larvae emerging from demersal eggs. This changed gradually to larvae hatching from pelagic eggs. Larvae from pelagic eggs dominated the ichthyoplankton assemblage in summer. A remarkably stable seasonality in terms of dominance patterns with recurring, season-specific fish assemblages was observed over the 3 years, despite substantial variation in environmental conditions such as a temperature difference of almost 20°C between summer and winter. The lesser sandeel (Ammodytes marinus), was the only species which showed significant fluctuations in abundance between the years. After removal of this species from the analysis, the dominance patterns of the remaining fish species were almost identical between years.     

Temporal patterns of breeding and larval settlement in a temperate population of the tropical anemonefish,Amphiprion clarkii

The breeding of the anemonefish,Amphiprion clarkii, and the settlement of larvae were observed in temperate Japan. Spawning had two peaks near the first and the third quarter of the moon in 1983, but did not in 1984. The clutch size varied greatly during the breeding season, with a peak from the second half of June to the second half of July. Adult fishes inhibited larval settlement. The proportion of disappeared 0-year olds to the total was higher in 1984 than in 1983. This difference might be mainly due to the difference in the degree of crowding in each sea anemone. The period of larval settlement closely paralleled the breeding season. The larvae settled abundantly around the first quarter and the third quarter of the moon in 1983, and did from the third quarter to the first quarter in 1984. In both years, the number of settled larvae was more from the full moon to the new moon than the new moon to the full moon. The daily pattern of larval settlement did not reflect that of breeding.     

Osmotic changes during the development of eggs and larvae of the lumpsucker, Cydopterus lumpus L.

The demersal eggs of Cyclopterus appear to osmoregulate like the pelagic eggs of cod and plaice. Unfertilized eggs in ovarian fluid exhibited ovoplasm osmolarities similar to those of adult blood and ovarian fluid (356–359 mosmol kg⁻¹). Yolk osmolarities remained virtually constant from fertilization and during development (356–366 mosmol), with a slight decrease near hatching (to 332 mosmol). Yolk and body fluids of larvae (338 mosmol) had osmoconcentrations similar to egg yolk values near hatching. Yolk osmoconcentration of unfertilized eggs remained unchanged during the first 12 h in sea water, with a slow increase thereafter. Fertilized eggs of bad quality cultures exhibited higher yolk osmoconcentrations than eggs of good quality. Cyclopterus eggs were found to develop normally and survive in 20–34%o salinity, larvae seemed to have the same salinity range.     

Early life history strategies of notothenioids at South Georgia

Antarctic notothenioid early life history strategies are examined in general and then for common species at South Georgia. Channichthyids, bathydraconids, artedidraconids and some nototheniids have large eggs 3·0–4·9 mm whereas other nototheniids and arpagiferids have smaller eggs 1·6–2·7 mm. At South Georgia the larvae of species with large eggs hatched between August (late winter) and late November (late spring) at 11–16 mm standard length ( L _s). Larvae of species with small eggs hatched mainly during October and December at 4.5–9 mm L _s. Most of the larvae of all species attain urostyle flexion between October and January, and develop to the end of the larval stage between November and May. The duration of the larval stage varies from 2 months in species with smaller larvae to 6 months for some of the species with larger larvae. Two nototheniid species develop to the early juvenile stage before a channichthyid and a bathydraconid that hatch around 2 months earlier. During their first winter, the early-juveniles of most species with large eggs are pelagic, whereas those of species with small eggs may be pelagic or demersal. Four groups of strategies are proposed based on egg size and the winter ecotype of the early-juvenile stage.     

Biology of the radiated shanny Ulvaria subbifurcata Storer in Newfoundland (Pisces: Stichaeidae)

Ulvaria subbifurcata is found on the north-east Atlantic coast of North America. Fish less than 55 mm preyed predominantly on copepods, while larger ones ate mainly nereids; polynoids and amphipods were eaten by all sizes. These four food items comprised the main food of the radiated shanny. Spawning occurred in May with the fish first spawning at age three to five. No conspicuous secondary sexual characters were observed. Females spawned once per year, producing a demersal egg mass. Males spawned up to four times and tended the egg mass (or masses). Incubation lasted 35–40 days, and the pelagic larvae settles on the bottom by August. Adult males tended to be larger than females of the same age but little difference was found sexually in the length–weight relationships. Ulvaria subbifurcata larger than 60 mm are nocturnal. This may reduce predation by, and competition with, other inshore benthic fishes.     

Cost of predation avoidance in young-of-year lake trout (Salvelinus namaycush): growth differential in sub-optimal habitats

Selection of habitat to avoid predation may affect the diet of young-of-year (YOY) lake trout (Salvelinus namaycush). YOY lake trout may use inshore habitat to avoid predation; this habitat may be sub-optimal for growth. To test this, YOY lake trout were penned in nearshore and offshore pelagic areas of two arctic lakes. Toolik Lake had a lake trout population, the other lake, S6, did not. YOY lake trout in Toolik Lake lost weight, but those offshore lost less weight. The YOY lake trout in Lake S6 gained weight and those offshore gained more weight. The primary diet item of the YOY lake trout in both lakes during this experiment was the zooplankter Diaptomis probilofensis; it was also one of the most abundant species. However, its density inshore in Lake S6 was similar to inshore and offshore densities in Toolik Lake. The increased availability of alternative zooplankton prey in Lake S6 may account for the growth differential of YOY lake trout in Lake S6 relative to Toolik Lake. Bioenergetic modeling of YOY lake trout suggests that growth similar to that in the offshore of Lake S6 would be necessary for successful recruitment. If the reduced zooplankton availability in Toolik Lake leads to the reduced growth of YOY in the inshore and offshore pelagic areas, then these fish will be more susceptable to winter predation/starvation. For YOY lake trout to survive in Toolik Lake they most likely shift to feeding on benthic prey before the end of their first summer. Dept. of Chemical Engineering     

Winter aspects of the ichthyoplankton community in Antarctic Peninsula waters

Summary Ichthyoplankton was sampled from the Antarctic Peninsula area of the South Polar Ocean in early winter (May and June 1986). A total of 153 eggs from two species and 1368 larvae or juvenile stages from 12 species were obtained. These included pelagic species, and demersal species with a long pelagic larval or juvenile phase. Most abundant were larvae of Pleuragramma antarcticum and Notothenia kempi, and eggs of Notothenia neglecta. The distribution of notothenioid and paralepidid larvae was apparently unaffected by ice cover, whereas myctophid larvae were confined to ice-free waters. Areas where newly hatched Chionodraco hamatus occurred coincided with dense aggregations of Euphausia superba (Krill) furcilia larvae which is a potential food resource during winter. The hatching of icefish larvae during winter is apparently independent of the seasonal production cycle. Epipelagic eggs of Notothenia neglecta were found during the spawning season, which suggests that eggs ascend to the surface after demersal spawning and that development takes place near the sea surface during winter. Larvae of Notothenia kempi were chiefly confined to shelf and slope waters to the west of the Antarctic Peninsula, with larger larvae found in coastal shelf areas. Pleuragramma antarcticum occurred in the coastal waters off the Biscoe Islands, in the Gerlache Strait, and in the northern Bransfield Strait. The smallest larvae were found in the northern Bransfield Strait, whereas those at the Biscoe Islands and in Gerlache Strait waters were larger and of a similar size. A cyclonic gyre to the west of the Antarctic Peninsula observed in the austral summer was likely to have affected the larval drift of Pleuragramma antarcticum and Notothenia kempi. Differences in the timing of spawning and hatching and the vertical distribution of these larvae will lead to different transport and spatial distribution patterns. It is hypothesized that early winter conditions do not imply severe limitations on the year-class success of larval fish. Dispersal and increased mortality may occur during the second half of the winter.     

Is speciation of demersal fishes in Lake Tanganyika restrained by physical limnological conditions?

Both Lake Malawi and Lake Victoria have many species of offshore demersal cichlids, most of which are stenotopic. In Lake Tanganyika there are fewer, more eurytopic, demersal species. These differences may be the effect of the physical regimes of the lakes. In Lake Victoria there is no permanent anoxic layer. In both the other lakes, water below 250 m depth is permanently anoxic and the seasonal cycle is dominated by upwelling. In Lake Malawi this is mainly derived from intermediate water containing free oxygen. In Lake Tanganyika upwelling involves hypolimnetic water, which may rise to within 80 m of the surface, causing great short-term changes in oxygen concentration over the depth range 50–250 m. This has inhibited the development of deep water species restricted to narrow depth ranges.     

Analysis of the demersal community of fish and cephalopods on the Agulhas Bank, South Africa

The demersal fish and cephalopod communities of the continental shelf and upper slope from 17 to 395m deep were studied during five annual cruises between Cape Agulhas and Port Alfred, South Africa. The cruises showed a consistent pattern of an inshore community (<100m), a shelf community ( c . 90–190m) and a shelf-edge/upper slope fauna (>200m). These groups were identified by dendrograms and multidimensional scaling cluster analysis, which supported on-board observations of catch variation with depth. Although the boundaries are not clearly defined, examination of physical features at the clustered stations suggests that depth, temperature and, to a lesser extent, oxygen concentration are important in the grouping. Occasional, apparently anomalous associations of inshore stations suggested that water temperature and oxygen may over-ride the normal depth distributions of the species groups. This intimates that patterns offish and cephalopod distribution may be dynamic and in part related to the physical parameters of the water body.     

Reproductive behaviour and early development of the Drysdale hardyhead, Craterocephalus sp. nov. (Pisces: Atherinidae), from the Alligator Rivers System, Northern Territory, Australia

Craterocephalus sp. nov. showed sexual dimorphism in body shape during the breeding season. Pairing occurred during daylight with a single release of eggs amongst submerged vegetation between sunrise and the early afternoon on the same day. The eggs were demersal, adhesive, spherical (0.87–0.96 mm diameter) and had 12 adhesive filaments (0.5–1.5 mm long) at the animal pole. Approximately 24 oil droplets (0.02–0.08 mm diameter) persisted throughout egg and larval development. Hatching occurred 155–160 h after spawning at 25–27° C.
The yolk-sac larvae were 3.85–3.95 mm notochord length at hatching and began feeding at the surface after absorption of the yolk (3–12 h after hatching). All fin rays were developed in 9.4 mm standard length fry, which moved from midwater to feed on the substrate. Aquarium reared fish first spawned at 30 mm s.L. when 165 days of age.
Features of Craterocephalus reproduction, as they relate to a specific survival strategy, are briefly discussed.     

Vertical distribution behaviour and its spatial variation in late-stage larvae of coral-reef fishes during the day

Daytime vertical distribution behaviour of settlement-stage reef-fish larvae in the upper 18 m was documented by diver observations of 497 pelagic larvae of 7 species 100-1000 m offshore of Lizard Island, Great Barrier Reef. Four species were studied on 2 sides of the island. Depth amplitude and depth frequency differed among species, locations and times. Four pomacentrids had modal depths in the upper 30-50% of the water column. A lutjanid and 2 chaetodontids had modal depths 0-2 m deeper than the deepest-swimming pomacentrid. On the leeward side, 6 of 7 species swam deeper and/or more variably offshore. On the windward side, 1 of 4 species swam deeper or more variably offshore. No larvae swam deeper than 18 m on the leeward side, but 31% of larvae of 3 species did so on the windward side. Three of 4 species swam deeper and/or more variably on the windward than leeward side. Vertical distributions in relatively shallow water are apparently strongly influenced by water-column depth and bottom type.     

Distribution and onshore migration behaviour of burbot larvae in Lake Constance, Germany

Larval abundance of burbot Lota lota in the pelagic zone of Lake Constance, Germany, peaked during April and remained at a constant but lower level until mid‐June. From the end of May onwards a significant diel vertical migration pattern was observed: while burbot larvae were distributed above as well as below the thermocline in the daytime with the highest percentages of abundance at water depths of 30 and 50 m (below the thermocline) on 5 and 25 June, they were concentrated from 2 to 15 m (above the thermocline) during the night. This migration pattern coincided with a significant decline in the food items preferred by burbot at this life stage, cyclopoid copepods. This phase of diel vertical migration lasted for c . 4 weeks until the burbot larvae suddenly disappeared from the pelagic zone. This indicated a short phase, lasting only a few days, of larval settlement into profundal benthic habitats. This was supported by a significant, abrupt decrease in the absolute growth rates of the larvae at day 56 after hatching. The day–night pattern of vertical migration observed could be assumed to be a transition phase from a pelagic to a benthic lifestyle, as reported for juveniles of several other, marine gadoid species.