Evidence that size-selective mortality affects growth of Atlantic cod (Gadus morhua L.) in the southern Gulf of St Lawrence

The effects of size-selective fishing mortality on the growth of Atlantic cod ( Gadus morhuu L.) in the southern Gulf of St Lawrence were investigated and compared between: (1) a period when fishing mortality was relatively high, growth was relatively rapid, and abundance low (1967–1972 year classes): and (2) a period when fishing mortality was lower, growth was slow, and density high (1977–1982 year classes). Cod first entered the fishery at age 3 during both periods. The 1967–1972 year classes (fast growing) were fully recruited to the fishery by age 5 or 6 and the fishery removed over twice as many fish from the lower than upper quartiles of length-at-age distributions for cod 4 to 10 years old (disproportionately high exploitation of slow-growing fish). In contrast. the 1977–1982 year classes (slow growing) did not fully recruit to the fishery until age 9 or 10 and the fishery removed four times as many fish from the upper than lower quartiles of the length-at-age distributions for 4- to 10-year-old cod (disproportionately high exploitation of fast-growing fish). The reduced mean lengths-at-age of the 1977–1982 year classes compared with the 1967–I972 year-classes is consistent with the different patterns of exploitation of fast- and slow-growing fish for the two periods.     

Reproductive biology of Vinciguerria nimbaria in the equatorial waters of the eastern Atlantic Ocean

Adult Vinciguerria nimbaria in an area of the Atlantic Ocean (0–5°N; 10–20°W), collected from November 1994 to February 1998, exhibited two different patterns in diel behaviour: 'typical' behaviour which consisted of large diel vertical migration and 'atypical', characterized by concentrations of schools that remained in the surface layers during the daytime. The total life span of V. nimbaria was estimated at 6–7 months. Females were mature when they reached 30·6 mm standard length ( L _S), which corresponded to an age of 85 days. Once the females were mature, spawning was continuous in the population as a whole, and V. nimbaria spawned continuously throughout the year. Spawning took place during two restricted times of the day: typical females spawned at dusk and atypical females spawned at dawn. Using the post-ovulatory follicles method, spawning frequency was estimated at 2 days. Batch fecundity was estimated at 1236 oocytes or 1230 oocytes g⁻¹ of total body mass, and egg size was 650 µm whatever the period of the year. The lifetime fecundity of V. nimbaria was estimated at 9000 eggs (109 000 eggs if mortality rate was not taken into account), and the maximum stock egg production of a theoretical cohort occurred at 37 mm L _S. Young adults thus contributed the most to the reproductive output for the survival of the population.     

Production of salmon and trout in a stream in Scotland

For three years the population size, rates of growth, standing stock, production and yield of all year classes of salmon and trout within three sections of a stream in Scotland were studied. Total salmon production as fresh weight per m² was 6.5 g in 1966, 10.6 g in 1967 and 11.1 g in 1968, and total trout production was 10.3g in 1966, 12.3 g in 1967 and 7.7g in 1968. Fish of 0+ and 1+ year old provided usually more than 90% of the total annual salmon production and 80 % of the annual trout production. Yield of salmon smolts (about 9 cm or longer after 2 years growth) per m² was 0.10 in 1966, 0.22 in 1967 and 0.15 in 1968. The smolt yield by weight was 29 % of the production of the 1966 year class of salmon and 16% of the 1967 year class. Numbers of trout of 9 cm or longer produced in the same time were higher and their weight was 60% of the total production of the 1966 year class and 32% of the 1967 year class.     

The populations of fish in the River Tees system on the Moor House National Nature Reserve, Westmorland

The age, growth, population densities and annual production of an isolated population of brown trout ( Salmo trutta L.) and bullhead ( Coitus gobio L.) were investigated over a 5 year period in Trout Beck in the northern Pennines. Additional data were obtained from two other sites; Great Dodgen Pot Sike and the River Tees.
Trout growth rates were low, particularly in Dodgen Pot Sike, but ages up to VIII were recorded. Bullhead growth rates were similar to those in the Tees at Cow Green (Crisp et al. , 1974), though the Moor House bullheads had a higher survival rate. Population densities of trout ranged from 0.10–0.22/m², with a fry contribution of 0.80%. The bullhead population in Trout Beck was sparse, generally 0.1/m², though an 0 group value of 0.46/m^a was estimated in 1967. At Tees Bridge values ranged from 2.5–7/8/m². Most trout matured by age III, though in Great Dodgen Pot Sike only half of the females were mature at age IV.
Annual production was estimated from Allen graphs. Trout production ranged from 1.02–3.50 g/m²/year, the fry contributing up to 51.5% of the total. A value of 0.48 g/m²/year was obtained for bullheads in the Trout Beck system in a year of good recruitment, whilst bullhead production at Tees Bridge was 7.43 g/m²/year, with fry and I group contributing 48.7 and 37.5% respectively. The high survival rate, irregular recruitment, and poor growth of these high altitude populations is discussed. Growth and reproductive peculiarities of the Great Dodgen Pot Sike trout are also discussed.     

A 27-year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown trout Salmo trutta , population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish >3 years old for period II. The gill-net catchability was correlated negatively with spawner biomass and number of trout >3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density-dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density-dependent recruitment of 1-year-old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.     

Growth and production of some important species of fish in a Mexican coastal lagoon system

In terms of biomass, the three dominant fish species in the Huizache-Caimanero lagoon system during 1975–76 were the mullet Mugil curema , the anchovy, Anchoa panamensis and the catfish, Galeichthys caerulescens . Peaks in the abundance of the secondary predators occurred at the end of the wet season (September-October), while pelagic forms and members of the centropomid-gerreid association (Warburton 1968a ) were most common during November-February and December-February respectively. Twelve species, including the ten having the highest overall mean biomass, were selected for growth and production studies. Lagoon-specific differences in the patterns of growth and recruitment of several species were observed. The total estimated annual fish production in Caimanero lagoon was 34·48 g m⁻² y⁻¹, with M. curema contributing 9·36 g m⁻², A. panamensis 9·24 g m⁻² and G. caerulescens 6·15 g m⁻² (maximum estimate). Mean turnover ratios (annual production: mean biomass) were 3·25 (demersal species), 8·44 (pelagic species) and 4·48 (all species). Implications for possible fishery and aquacultural expansion are discussed.     

Primary production and respiration of the phytoplankton of the Rivers Thames and Kennet at Reading

During the period April 1967-ApriI 1968 the phytoplankton production and respiration of the River Thames and its tributary, the River Kennet, were measured at approxi-mately 2-week intervals using the light and dark bottle technique. Concentrations of chlorophyll and pheopigment were determined weekly. On fourteen occasions sets of light and dark bottles were rotated in a specially designed apparatus, and production and respiration values obtained were found to be 1·38 ± 0·31 times higher than in stationary bottles at identical depths over the same period. There was little horizontal, vertical or diurnal variation in chlorophyll concentration showing that the water was well mixed. Peaks of chlorophyll were found in spring, summer and autumn in the Thames (max. 219 mg/m³) but there was very little variation in the Kennet (max. 38·2 mg/m³). In both rivers lowest concentrations were found during winter. Pheo-pigment concentration was low in both rivers for most of the period although in the Kennet this represented on average 50% of the pigments present. In the Thames a peak of pheopigments(1·33–5 mg/m³) was associated with the autumnal bloom and repre-sented 61 % of the total pigments. No pheopigments were detected during the spring bloom. The average concentration of suspended organic matter was identical in both rivers but in the Thames over 25 % was due to phytoplankton and in the Kennet almost 95 % was non-algal. In the Thames, net oxygen production reached a peak in May (10·81 gO₂/m²/day) and was negative from November to February (min. ?0·45 gO₂/m²/day). In the Kennet, maximum production also occurred in May (0·85 gO₂/m2) but was negative from the middle of May until the following March. The average annual net production was 1250 and ?78 g O₂/m² in the Thames and Kennet respectively. Respiration rates showed similar fluctuations being 4·59 g O₂/m²/day in spring in the Thames to 0·09 g O₂/m²/day in November. The Kennet was almost always lower (1·05–0·34 g O₂/m2/day. The average annual respiration was almost three times higher in the Thames than in the Kennet (641–228 g O₂/m²). Various factors which might influence production are discussed. The average net efficiency ofthe Thames phytoplankton fell within ranges described from other rivers. Net efficiencies ofthe Kennet were almost always negative. In the Thames it appeared that net production could be explained as a function of solar radiation, chlorophyll concentration and euphotic depth.     

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, movements, and mortality of anadromous arctic char, Salvelinus alpinus L., in the Cumberland Sound area of Baffin Island

During 1972 the downstream movement of anadromous artic char, Salvelinus alpinus L., from two rivers flowing into Cumberland Sound, Baffin Island, began during the middle of May and was completed within 2 weeks. This movement took place during both the day and night. Upon reaching saltwater, many char older than 9 years (longer than 20 cm) began to migrate along the shore of the Sound at a rate of 0–6–0-9 km/day with the result that the average maximum distance travelled from the natal river was 40–50 km. Char 6–9 years old, 10–20 cm in length, remained much closer to the rivers while those younger than 5 years never entered saltwater. During the period of saltwater residence, the fish frequently moved into the intertidal zone and freshwater. Maturing fish of both sexes were occasionally found in saltwater. The upstream migration to the lakes began during the second week of August and was completed within 5–6 weeks. Females tended to ascend the rivers before males, although fish of all different size and age migrated upstream simultaneously. Migration occurred during both the day and night. The maximum distance anadromous char moved from saltwater was 40 km. The char were found only in freshwater between May and September and were normally located near the bottom of lakes at a depth of 15–40 m. During this period, large scale movements were probably very limited. Average annual mortality was estimated at 16.0% per year with the highest rates (25–30%) occurring at ages 10 and 15–17 years. Many fish probably died of old age but physical deterioration during and after spawning was also an important cause of death.     

Estimation of the number of brown trout Salmo trutta (L.) in Loch Leven, Kinross, Scotland

The numbers of adult trout in Loch Leven were estimated in April each year 1968–71 by tagging fish caught by seine net, and estimating the proportion of the total stock tagged from examination of the angling catch during June-August. The vulnerability of tagged fish before June and after August was higher than that of untagged fish. Tag losses, estimated by double marking, were 2.15 % over a whole angling season. The reporting rate of tag recaptures varied within and between years from 0.43 to 0.71. Differential mortality of tagged and untagged fish was unimportant, except in two subsidiary experiments when fish were tagged in June and August, when handling losses reached 2.7%. With adjustment for these measured errors, the stock of trout beginning their third or more years in the loch in April, fell from 126 665 in 1968 to 52 337 in 1971.     

The production of brown trout, Salmo trutta in tributaries of the Upper Wye, Wales

The population density, age structure, biomass, growth and production of brown trout were investigated in four tributaries of the upper River Wye. The populations at each site were largely maintained by immigration from nursery areas. Abundance of separate year classes at sites on the three largest tributaries reached a peak at age 2+. On the smallest stream numbers reached a peak at 1+. Recruitment occurred throughout the year but decreased with age of year classes. Maximum O+ densities ranged from 0.04 to 0.89 m⁻², and >0+ densities from 0.13 to 0.59 m⁻². Average total biomass in 1975 ranged from 2.6 to 14.2 g m⁻². Within the study sites annual trout production in 1975 ranged from 2.9–19.7 g m⁻². Production values were dependent on age structure and population mobility at the study sites. In the three largest streams 2+ and 3+ fish contributed 66.3–88.3% of total production whilst 1+ and 2+ fish contributed 74.5–84.5 % of the total in the smallest stream. The mobile (non-resident) component of the population accounted for up to 60–70% of production at certain times of the year, but over the year (1976) accounted for =30 % of total production. The resident component of the highest annual production value (19.7 g m⁻²) was estimated to be between 15.0 and 18.2 g m⁻².     

Bothriocephalus scorpii (Müller) (Cestoda: Pseudophyllidea) in turbot Scophthalmus maximus (L.) and brill S. rhombus (L.) from the southern North Sea

Turbot (11–70 cm long) and brill (21–70 cm long) were collected in the summer of 1968 north of the Brown Bank, Texel Hole and were infected by Bothriocephalus scorpii. Infections occurred even in the first year of life of the fish. In turbot, with increasing length, the average weight of tapeworm rose from 0–6 to 14-9 g/flsh. Brill were less commonly infected and then only slightly. The intermediate host of the tapeworm seems to be Gobius species, upon which juvenile turbot and brill prey most.     

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.     

Time of start of spawning in Atlantic cod (Gadus morhua) females in relation to vitellogenic oocyte diameter, temperature, fish length and condition

Spawning time of female Atlantic cod ( Gadus morhua L.) can be predicted to within 3 days using vitellogenic oocyte diameters measured over the last month before spawning. Simulations show that a l°C drop in temperature during vitellogenesis delays spawning by about 8–10 days. Similar results were obtained with fish on low ration for 4–8 months before spawning. Moderately fed females held over two to four consecutive spawning seasons demonstrated a significant delay (⋍3–40 days) in the date of first spawning as the fish grew older and more fecund. Forecasting studies on spawning migrating Arcto-Norwegian cod gave no evidence of a significant effect of fish length on the time of spawning. Warmer water temperature during vitellogenesis for the larger fish appears to compensate for the negative effect of fecundity On spawning time.     

Ground vegetation biocontent and net production in a deciduous woodland

Summary Biocontent of the above-ground parts of ground vegetation in an alder-birch woodland was between 75 and 581.3 Kcal/m² (16.05 and 137.70 g/m²). Species peaks did not necessarily coincide with the highest values for total ground vegetation. Ground vegetation litter biocontent was 1848.4 Kcal/m² at the beginning of the study (April 1966) but rapidly fell to near 700 Kcal/m² and remained near this level until April 1968. Net production by the above ground parts of the ground vegetation was 1016 Kcal/m²/yr (277.86 g/m²/yr) and 1142 Kcal/m²/year (280.20 g/m²/yr) in 1966–1967 and 1967–1968 respectively, measured as mortality plus net biocontent changes. These estimates were compared with those based on biocontent changes. Ground vegetation net primary production was not much smaller than tree litter fall (circa 1450 Kcal/m²/yr).     

The energy budget of the brown trout population of øvre Heimdalsvatn

Energy pathways of a natural brown trout Salmo trutta population were constructed. The main subjects taken into account were: the biomass, calorific values and production of the population, natural mortality and annual yield, energy intake, calorific values and composition of the food, efficiency of food absorption, spawning and migration of young trout. The specific energy value of trout increased with increasing weight of the fish. An energy decrease of almost 50% was demonstrated for the spawning trout of both sexes, and the calorific values prior to spawning were not reached before the next summer. Energy intake was estimated to 6.87, 10.31, 8.42 and 9.15 kcal m⁻² lake surface for the years 1969, 1970, 1971 and 1972 respectively. Although a great number of food items were found in the trout stomachs, the four groups Gammarus lacustris, Lepidurus arcticus , Trichoptera and Chironomidae accounted for 2/3 of the total consumption. The efficiency of food absorption varied between 42 and 53%. The mean age of trout entering the lake from the nursery streams was ca. 3 yr. The annual amount of this immigration was estimated to be 0.28 and 0.24 kcal m⁻² lake surface for 1969 and 1970 respectively. Annual production of trout in the take was estimated to be 1.46 and 2.66 kcal m⁻² respectively for the same years. For both these years balance was found between biomass at the beginning of the year plus immigration of young and total production, minus catch by man, natural mortality and spawning on the one hand and biomass at the end of the year on the other hand.     

Reproductive cycle and sex inversion in razor fish, a protogynous labrid in the southern Mediterranean Sea

The reproductive biology of the Mediterranean razor fish Xyrichthys novacula was investigated by demographic data and histological analysis of the female, intersexual and male gonads. Specimens were collected by bottom trawl on a monthly basis between June 2000 and July 2001 in a sandy bay in southern Thyrrenian. Gonad histology confirmed that the Mediterranean razor fish is a monandric, protogynous hermaphrodite. Females reached first sexual maturity at 100 mm ( L _T) and the estimated mean L _T at first maturity ( L ₅₀) was 125 mm. Females exhibited asynchronous ovarian development and multiple ovulations occurred over the spawning period. Vitellogenesis started in early May and spawning occurred from late May until late September. Sexual transition involved a large‐scale atresia of all oocyte stages and a massive degeneration of ovarian tissue followed by primordial germ cells proliferation. Sex change began at spawning time (June) but transitional individuals tended to cluster at the end of the reproductive period (September). They accounted for 17·1% of the population sampled and were found in a broad size range (105–150 mm L _T).     

Production and food consumption of predatory fish in the Vistula River

Vital statistics for six predatory fish species occurring in the Vistula River were estimated. The mixed, six species population, in its predatory phase, had an average biomass of 0-96 g/m2 and produced 043 g/ma. Less than 45% of annual production was exploited by man and eggs shed amounted to about 10% of the production. Annual consumption by the population amounted to about 3 -44 kcal or g/ma. Accuracy of all estimates is low.     

Habitat, reproductive biology and size composition of Parequula melbournensis, a gerreid with a temperate distribution

Trawling was carried out over sandy substrates in the shallow (5–15m) and deeper (20–35m) waters of four regions along 250 km of the lower west coast of Australia, during seven consecutive seasons. This yielded 32 752 individuals of the gerreid Parequula melbournensis , which constituted c. 42% of the total number of fish. Densities of P. melbournensis were greatest in the most southern region, reaching a seasonal maximum of 835 fish ha⁻¹: at one site in that region. Since P. melbournensis is restricted largely to the southern coastline of Australia, it has a temperate rather than subtropical or tropical distribution and thus is not a typical gerreid. Furthermore, unlike most other gerreids, it does not spend part of its life cycle in either estuaries or nearshore marine waters. The maximum total length of P. melbournensis was 175 mm, with the length at maturity ( L ₅₀) being 115 mm in females and 121 mm in males. No clear monthly trends were exhibited by gonadosomatic indices, the prevalence of mature ovaries and the oocyte size-frequency distributions of female P. melbournensis , and no clear and consistent modes were observed in length-frequency data for this species. These strong indications that spawning occurs throughout the year were substantiated by the occurrence of post-ovulatory follicles in the ovaries of large fish in all months but August, and by the presence in that month of advanced yolk granule oocytes in some ovaries, which implies that spawning was imminent. The spawning of P. melbournensis throughout the year contrasts with the far more restricted spawning periods recorded for other teleosts in the same temperate Australian waters. In this respect, P. melbournensis exhibits the characteristics of the essentially tropical family to which it belongs. Annuli, which could be detected on the otoliths of c . 40% of fish, suggest that the majority of P. melbournensis Were <3 years old.     

An experimental study on the natural removal of dead trout fry in a Lake District stream

The chief objective was to discover why few moribund or dead Salmo trutta fry were observed on the stream bed when mortality within the stream was known to be high ( c . 13 000 dead fry year ⁻¹ for whole stream). Newly dead fry were placed in 20 boxes embedded in the stream bed (20 fry of known total weight per box) and arranged in pairs with one box open and one closed. One pair was removed every 2 days, the fish remains being weighed and the invertebrates in the open box being identified and counted. The experiments were performed from late April to early May in 1967, 1968, 1969 and the results were similar in each year. Both wet and dry weights of fry decreased exponentially but the rate of decrease was much higher in the open boxes; detectable fish remains were about 55% of initial weights after 20 days (end of experiment) in closed boxes but zero after 16 days in open boxes. Invertebrate scavengers were responsible for the higher loss rates in the open boxes and showed a definite succession with caddis larvae and carnivorous stonefly larvae dominant at first, but then being replaced by detritivorous stonefly larvae and freshwater shrimps. These experiments show clearly why dead fry disappear rapidly from the stream bed.