Depth distribution and biological characteristics of the European eel Anguilla anguilla in Lough Ennell, Ireland

Using a longline survey, a total of 196 European eels Anguilla anguilla were collected at different depths in Lough Ennell (maximum depth 30 m), central Ireland. The catch per unit of effort of A. anguilla that were caught from 1 to 25 m depths was lowest at 0·5–5·0 m and greatest at the deepest depth range (22·5–25·0 m). Sub-samples of A. anguilla from depths of <15 m showed little or no difference in size, sex ratio, age, growth rate, condition factor, length–mass relationship, gonado-somatic index, fin index or eye index with fish from depths of >15 m. All fish examined were female yellow-phase A. anguilla that had ages from 7 to 20 years (mean ± s . d . = 10·3 ± 2·9 years), with growth rates from 24·0–60·8 mm year⁻¹ (mean ± s . d . = 40·7 ± 8·5 mm year⁻¹). Variations in the growth rates were greater in the shallow group than that of the deep group. This study suggested that deeper regions are important feeding habitats for A. anguilla and that fish in this lake were growing moderately fast compared to similar habitats and areas in the species' range.     

The ability of Tilapia mossambica (Peters) to enter deep water

Juvenile Tilapia mossambica (Peters) are extremely tolerant of depth and can compensate to a depth increase of 0 to 33 m in a few minutes. As the fish grow they become less able to compensate to deep water. At a total length of 15–18.5 cm small adult male T. mossambica can compensate to 20 m whereas females can only compensate to 13 m. Large adults (20–25 cm) of both sexes compensated to 11.5 m (range 9.5–13 m). Adult fish require 4 to 7 days for maximum depth compensation at 22°C. Analysis of the oxygen content of the swimbladder indicated that this gas accounts for over half the gas in surface equilibrated fish. Oxygen secretion into the swimbladder appears to be the mechanism of equilibration at depth.     

Age estimation, growth, maturity and distribution of the roundnose grenadier from the Rockall trough

This paper summarizes the history of commercial exploitation of roundnose grenadier Coryphaenoides rupestris in the North Atlantic. Length frequencies of C. rupestris in 1993, from 400 to 1200 m on the slopes of the Rockall trough indicate a reduction since the 1970s in the modal length of fish found at 700–1000 m. Ages ranged from 2 to 50 years for males and 2 to 60 years for females, with most between 10–38 years. Females attained a greater asymptotic pre-anus length ( L _∞=19.5 cm) than males ( L _∞=15.5 cm) and had a greater weight for a given age (male W _∞=761g, female W _∞=1132g). This species may have a protracted spawning period. Using pre-anus lengths, 50% of male fish were mature at 10 cm (ages 8–10) while 50% of female fish were mature at 12 cm (ages 9–11). At the greatest depths sampled the length frequency of fish was bimodal with a hiatus between 9 and 11 cm (ages 8–12). Highest catch rates occurred on the Donegal slope in September at a depth of 800–1000 m.     

Spatiotemporal variation in the population structure of the European hake in the NW Mediterranean

The bathymetric distribution of Merluccius merluccius was studied as a function of length, age and maturity of specimens caught by commercial trawl and longline in Different seasons. Males matured first at 28·8 cm (3 years old), and females at 38·0 cm (3·5 years old). Reproductive activity was noted practically throughout the year with its most pronounced spawning peak in the autumn. Most fish <37–40 cm were males and >46 cm were females. Specimens occurred between 50 and 750 m depth, although density was low at >400 m. Adults were found at all depth strata studied. Recruits and juveniles were limited to inshore waters <400 m, most were found between 100 and 200 m. Spring and summer were the preferred seasons for recruitment; although for both seasons there was some interannual variation. Adult distribution also varied, according to the season. Young adults were spread over the entire depth range, with the biggest ones concentrated at the edge of the shelf (150–350 m), especially in autumn and winter. The main spawning peak coincided with this concentration of adults suggesting that spawning occurred in autumn/winter at the edge of the shelf.     

Age and growth of Mediterranean albacore

Estimated ages of 1136 individual albacore Thunnus alalunga (57–92 cm L F) from the Aegean and Ionian Sea ranged from 1⁺ to 9 years. Males grew faster and reached a greater size and age than females. No significant differences were found in the mean lengths at estimated ages between the two sampling areas. The von Bertalanffy growth model was fitted to mean lengths of estimated ages of individual fish and estimated growth parameters for the combined sexes were: L _∞=94·7 cm, K =0·258, to=–1·354 years. Significant differences were found when the Mediterranean albacore growth parameters were compared with those determined for Atlantic Ocean albacore. It is not possible to determine if the differences in growth rates for the two populations are phenotypic or genotypic at the present time.     

Population structure, growth and reproduction of roughhead grenadier on the Flemish Cap and Flemish Pass

Roughhead grenadier Macrourus berglax were collected during the Flemish Cap bottom survey (1991–2001, NAFO division 3M, depth of 200–720 m) and on commercial vessels (1998–2000, NAFO divisions 3L, M and N, depth of 700–1700 m). The abundance of roughhead grenadier, as well as their average size, increased with depth. Age and length composition of the catches showed clear differences between the sexes. The proportion of males in the catch declined in the larger length classes and there were no males in the largest length classes (25 cm). There were differences in the growth trajectories of the two sexes; both sexes grew similarly up to 9 years, but the male growth was slower thereafter. The pre-anal fin length at first maturity ( L _PA50) for roughhead grenadier females was 28·5 cm, whilst the age at 50% maturity was 15–16 years. Individual total fecundity varied between 8522 and 61 844 oocytes. Macrourus berglax had a prolonged life cycle and multi-aged population structure, with differences in growth and mortality between males and females, slow maturation and low fecundity.     

Reproductive biology of gag in the southern Gulf of Mexico

Aspects of the reproductive biology of gag Mycteroperca microlepis in the southern Gulf of Mexico were studied by following seasonal variations in the gonado‐somatic index and through histological examination of gonads. Gag were collected from inshore and offshore waters of the Campeche Bank, Yucatan, Mexico, between April 1996 and December 2001. This species is a protogynous hermaphrodite, and appeared to be depth‐size distributed. The smallest gag (9–49 cm L _F) collected were all juvenile females, and were caught in inshore waters (1–10 m depth), while the largest (49–116 cm L _F), mainly adult females, males and transitionals, were captured in offshore waters (33–167 m depth). Overall the offshore male to female ratio was female‐biased (1 : 3·3) and differed significantly from unity. The species spawns at depths of c . 50–53 m, from early winter to mid‐spring, with peak spawning activity occurring between January and March. Fifty per cent of females reached first maturity at 72·1 cm L _F. At 103 cm L _F, 50% of sampled females had changed into males. Gag can be considered a monandric species, and sexual transition for this grouper seemed to occur in fish distributed within a narrow size range (85–111 cm L _F). The results are compared with those of other authors for gag stocks from the south‐east Atlantic coast of the U.S.A. and the north‐east Gulf of Mexico.     

Film and television in fishery research at Llyn Tegid (Bala Lake), Wales

A 35 mm Robot Royal camera and an electronic flash unit were used at 20–40 m depth in an attempt to identify fish shown on echo-sounder traces. No photographs were obtained. An 8 mm Eumig automatic electric-drive cine camera system was used with daylight at 2–8 m depth to test if the equipment served as a deterrent to the fish. Fish were filmed and their behaviour was unaffected by the presence of the equipment. A simple closed-circuit television system was used with daylight at 0–3 m depth. Low contrast, inherent in the particular equipment, prevented its use at greater depths. Meaningful observations of perch were possible. The use of television camera tubes sensitive to extremely low levels of daylight, and of infra-red sensitive tubes with an infra-red light source is briefly discussed. It is concluded that such systems might help to identify problems of netting technique and assist with observations of fish behaviour.     

Behavioural thermoregulation by subyearling fall (autumn) Chinook salmon Oncorhynchus tshawytscha in a reservoir

This study investigated behavioural thermoregulation by subyearling fall (autumn) Chinook salmon Oncorhynchus tshawytscha in a reservoir on the Snake River, Washington, U.S.A. During the summer, temperatures in the reservoir varied from 23° C on the surface to 11° C at 14 m depth. Subyearlings implanted with temperature-sensing radio transmitters were released at the surface at temperatures >20° C during three blocks of time in summer 2004. Vertical profiles were taken to measure temperature and depth use as the fish moved downstream over an average of 5·6–7·2 h and 6·0–13·8 km. The majority of the subyearlings maintained average body temperatures that differed from average vertical profile temperatures during most of the time they were tracked. The mean proportion of the time subyearlings tracked within the 16–20° C temperature range was larger than the proportion of time this range was available, which confirmed temperature selection opposed to random use. The subyearlings selected a depth and temperature combination that allowed them to increase their exposure to temperatures of 16–20° C when temperatures <16 and >20° C were available at lower and higher positions in the water column. A portion of the subyearlings that selected a temperature c. 17·0° C during the day, moved into warmer water at night coincident with an increase in downstream movement rate. Though subyearlings used temperatures outside of the 16–20° C range part of the time, behavioural thermoregulation probably reduced the effects of intermittent exposure to suboptimal temperatures. By doing so, it might enhance growth opportunity and life-history diversity in the population of subyearlings studied.     

Growth of dark chub, Zacco temmincki (Cyprinidae), with a discussion of sexual size differences

I examined the annual and seasonal growth of dark chub, Zacco temmincki, in a Japanese river. Investigation of opercular rings showed that the fish reached a maximum age of 8 years. There was no significant sexual size difference at younger ages (3–5 years), but males were larger than females at older ages (6–7 years). Annual increments of length and weight for males that were recaptured were also larger than those for females. The fish spawn from June to August. Females grew for a short period from April to May, but males on average grew for a longer period from April to August. There was no sexual difference in growth rate except during the spawning period. Annual growth rate was negatively correlated with fish length in each sex. The sexual size differences at older ages of the fish might be due to the polygynous mating system in which most mature males could not obtain females and invested for somatic growth in the spawning period, and a short growing season that was overlapped considerably with the spawning period.     

Effects of age, sex and abundance on the bathymetric pattern of American plaice in the southern Gulf of St Lawrence

American plaice Hippoglossoides platessoides density in September in the southern Gulf of St Lawrence was strongly related to depth, with peak densities at intermediate depths (70–90 m). Bathymetric pattern was similar for all ages of plaice (3–12 + year) and varied little among years. Densities were highest at similar depths for males and females, but depth distribution tended to be more spread out for females. Depth distribution was not significantly density dependent except for 10-and 11-year-old males and for females aged 12 years and older. In the three significant cases, depth distribution tended to spread out as abundance increased. Water temperature and depth were closely linked in the southern Gulf, but plaice depth preferences could not be attributed solely to temperature selection.     

The growth, development and reproduction of a deep sea cumacean (Crustacea: Peracarida)

A single sample of over 1000 individuals of Leuconjonesi Bishop, 1982, taken at a depth of about 1500 m on the continental slope off Surinam, was studied. The carapace length of individuals in the first postmarsupial instar was already about half that of brooding females. Growth increments at early moults were of 20–25. Males reached the final morphological form in the sixth postmarsupial instar. A prepuberty moult was recognized between the third and fourth male instars. Females reached the preparatory stage in the fourth postmarsupial instar. Brooding condition occurred for the first time after the next moult. The brooding form could then alternate at successive moults with an inter-brood form, resembling the preparatory female, with small, non-overlapping oostegites. Preparatory and inter-brood females were morphologically separable; vitellogenesis was restricted to these instars. Broods were of 6–12 young. The individuals in each brood probably completed marsupial development, and were released, one by one. The copepod parasite Sphaeronella infested 10", of brooding-form females.     

Reproductive biology of anadromous arctic char,Salvelinus alpinus (L.)? in the Cumberland Sound area of Baffin Island

The ovaries of anadromous arctic char, Salvelinus alpinus (L.), captured during 1973 from four high arctic rivers on Baffin Island started to increase in weight early in July and were at their heaviest in mid-September just before spawning. At this time, 40 cm (615 g) char from two rivers contained ovaries weighing 17.0 g (dry) which differed significantly from the 22.5 g gonads of comparable char from the other two areas. Gonadal development in male fish began in June and the diameter of testes at full maturity (3.0–4.0 cm) was similar regardless of river system. Fecundity of comparable fish varied significantly among the rivers with char 40 cm in length containing 1400–2400 eggs while the corresponding values for 60 cm (i.e. 2050 g) char ranged from 4000–5500. The diameter of both immature and mature eggs was similar regardless of system. Values for immature eggs averaged 0.64 mm in 40 cm fish and 1.60 mm in 60 cm specimens. Diameter of mature eggs, on the other hand, was considerably greater (3.2 and 4.3 mm, respectively). Maturity was first reached at age 10 (i.e. 35 cm) in both males and females and maturing fish of both sexes were not always restricted to fresh water. Redd construction occurred at a depth of one to at least 11 m at a temperature of 0.5–4.0° C. The percentage of eggs retained in the body cavity after spawning varied from 0.22–1.50 and the mortality rate of spawned eggs was extremely low.     

Life history and distribution of the small south-western Atlantic octopus, Octopus tehuelchus

Although this species has been reported from shallow waters down to 90 m depth, knowledge is almost entirely based on intertidal samples. In this study both intertidal and subtidal samples were taken during 1982–1987, in northern San Matias Gulf (41° S, 63° 30' W). This is a large-egged (eggs: 9–12 mm long ×–5 mm wide, stack of 4–6 mm long) and small-sized (up to 150 g) octopus. Egg laying occurs between autumn and winter. Embryonic development takes about four months (water temperature:–19°C). Large hatchlings (DML: 5·64 mm, TL: 14·23 mm, TW: 0·139 g) emerge over spring and early summer, and development is direct. Maximum size is reached after 17–18 months; mating takes place in summer. Females reduce their feeding activity when they reach maturity, and cease eating while brooding. Mean life-span is two years, but some individuals (mostly females) may live up to three years. Females approaching the beginning of the brooding period move to the subtidal zone, where males outnumber females until the end of summer and females (mostly brooders) then outnumber males. In the intertidal zone sex ratio was 1:1 from December to late March, but in April males outnumber females.
These life-history traits are compared with those of other large-egged octopuses and are discussed in relation to environmental conditions prevailing in the San Matias Gulf.     

Movements of adult Atlantic salmon through a reservoir above a hydroelectric dam: Loch Faskally

Movements of adult Atlantic salmon were determined as they migrated through Loch Faskally, a 4-km long hydroelectric reservoir in North-east Scotland. The horizontal and vertical movements of four salmon were monitored for periods of 4–7 days using depth-sensing acoustic transmitters in June–July 1995. Each fish began sustained directed upstream movements within 5·5 h after release at swimming speeds of 0·15–0·40 bl s⁻¹. Three fish reached the head of the loch after 7·25–17 h, but then returned downstream. The four fish remained in the upper half of the loch for 15–51 days, making localized movements. Mean depths of fish were 3·7–4·0 m (max 20·7 m). Two fish were recorded at significantly shallower depths at night than during the day. Departure from the loch coincided with periods of high water flow into the reservoir. In May–July 1996, 17 radio-tagged salmon entered Loch Faskally and reached the head of the loch in 3 h–5·8 days (mean 39 h). The durations of stay in the loch varied from 3 h 50 min to 67·4 days (mean 10·9 days). Only two radio-tagged salmon left the loch under conditions of high water flow into the loch.     

The biology of the scaldfish, Arnoglossus laterna (Walbaum) on the west coast of Scotland.

The biology of a Scottish population of the small bothid flatfish, Arnoglossus laterna , was studied from January 1975 until September 1976. The data were taken from monthly samples totalling over 500 fish trawled in 18–36 m on a soft mud bottom. Otoliths were used for age determination and a growth curve was constructed which showed that most growth occurs in the first 2–3 years of life. The maximum age recorded was 8+ years. The fish first mature sexually in their second year at a standard length of 6–7 cm and the short spawning season lasts from the late June to August. Fecundity is length-dependent and the relationship could be described by the regression equation: log fecundity = 3·3472 log standard length (mm) -2·1064. The diet consists mainly of decapod crustaceans (particularly crangonid shrimps), polychaetes, mysids and small fish.     

Distribution and reproduction of the southern lantern shark from New Zealand

Distribution, population structure and reproduction are described for the southern lantern shark Etmopterus granulosus at the Chatham Rise, New Zealand. Depth of capture for E. granulosus ranged from 744 to 1420 m, with highest catch rates between 800–1200 m. More than twice as many females as males were captured, and the majority of sharks caught were mature, indicating that there may be segregation according to sex and size class. Only 10 of 492 female sharks captured contained ova in uteri, and none contained embryos. The absence of pregnant females suggests that they move to another area or depth prior to pupping. Size of sharks captured ranged from 20·0 to 78·8 cm total length. Females began to mature at 62 cm total length, and males at 52 cm. There was no evidence of a seasonal reproductive cycle. Ovulation appeared to occur when ova reached a diameter of 40–45 mm. The average number of ova in mature females was 12·7. This information is crucial for assessing the impact of fisheries on E. granulosus populations.     

A population study of Barbus barbus L. in the River Severn, England

Scales from 7071 barbel from the River Severn were used for age and growth determinations. The scales showed clear annual checks which were laid in May when most fish spawned. Scale structure and length frequency distributions showed an almost total absence of 1968 year class barbel. Growth was significantly faster in some sections of the sampling area than in others. Back-calculated growth analysis showed a systematic decrease in length for each age for females and males in the 1961 to 1967 year classes. Females grew significantly faster than males after four years of age and reached greater ultimate size. There was a preponderance of males between the ages of III and VII and a dominance of females in older age groups. The overall sex ratio of males to females was 2–41: 1. A distinct seasonal growth pattern was shown.     

Biology of the annular seabream, Diplodus annularis (Sparidae), in coastal waters of the Canary Islands

The biology of the Canary Islands annular seabream Diplodus annularis (Linnaeus 1758) was studied from samples collected between January and December 1998. Fish ranged from 82 to 209 mm total length in size and from 8.7 to 137.1 g in weight. The mean length showed an increase with increasing water depth. Males showed a negative allometric growth and females isometric growth. The species was characterized by protandric hermaphroditism. The overall sex ratio was unbalanced in favour of males (1 : 0.79). The reproductive season extended from January to May, with a peak in spawning activity in March–April. Males reached maturity at a smaller length (103 mm, 1-year-old) than females (128 mm, 2-year-old). Fish aged 0–6 years were found. The von Bertalanffy growth parameters for all individuals were: L_∞=247.9 mm, k=0.268 years^–1, and t₀=–0.879 years.     

Seasonal changes in the habitat use and movements of adult European grayling in a large subarctic river

In late summer (13 August–13 September 1998), at water temperatures of 12·0–15·7° C, grayling ( n =14) stayed mainly in the riffle-section where they were captured in a large regulated river in northern Finland, moving little between consecutive days. In autumn (2–30 October 1998), at 1·7–6·7° C, the fish ( n =16) migrated to potential overwintering sites 0–14 km up- or downstream by mid October, moving mainly short distances thereafter. The daily movement rates, and the total ranges covered by the fish in late summer and autumn were 54±32 m (mean± s.d ) and 1053±1636 m, and 190±168 m and 3135±1850 m, respectively. In autumn the fish used deeper habitats (most suitable range 150–400 cm) with lower current velocities (20–80 cm s⁻¹) and finer bottom substrata (mainly sand) than in late summer (depth 100–325 cm, velocity 30–110 cm s⁻¹, and cobble-boulder substrata).