Arild Folkvord

Effects of initial size distribution, day length, and feeding frequency on growth, survival, and cannibalism in juvenile Atlantic cod (Gadus morhua L.)

Abstract Two experiments were carried out with juvenile Atlantic cod, Gadus morhua L., in order to examine the effects of initial size distribution on growth and survival. The initial size distributions significantly affected growth and cannibalism rates among 0.6-g cod but not among 10-g cod. Cannibalism occurred among 0.6-g cod when the max:min weight ratio exceeded 3.5:1 and was the main mortality factor when the ratio was above 8.5:1. Total mortality rates ranged from 0.25 to 10.0% per day. Small differences in initial coefficient of variation (CV, weight) among graded groups (28.0 vs. 37.5%) resulted in significantly different rates of cannibalism (0.03 vs. 0.39% day−1). Mortality was size-dependent, with survival of the smallest fish being very poor in the groups with the largest size variation. The largest fish, presumed to be the cannibals, grew faster than smaller cod. Average growth of 0.6-g cod was about 4.0% per day. No cannibalism was observed among 10-g cod, and those fed moist feed grew faster than those fed dry feed (2.8 vs. 2.5% day−1). In a third experiment the growth rate of graded 0.9-g cod was higher with 24 h of light per day than with 16 h of light per day (2.7 vs. 2.5% day−1). No differences were found in growth or mortality rates using feeding frequencies from once every minute to four times per day. Final CV (weight) was generally independent of initial CVs over 45% and was, therefore, not a good measure of past mortality rates.

Growth, oxygen consumption and activity of juvenile turbot (scophthalmus maximus L.) reared under different temperatures and photoperiods

Abstract Juvenile turbot (5–125 g) were reared under two experimental temperatures: 10°C and 16°C, and three experimental photoperiods: LDN (natural photoperiod), LD 16:8 (16 h light: 8 h darkness), LD24:0 (continuous light), to study effects of temperature and photoperiod on growth, activity and oxygen consumption. Growth was strongly affected by temperature and was higher at 16°C than at 10°C. Continuous light had a growth-promoting effect at 10°C from mid-December to late March, while at 16°C this effect was restricted to December and January. A seasonal change in the condition index was found. The groups reared in continuous light had higher condition indices in winter. The experimental groups held at 16°C had higher O2 consumption than those at 10°C. The LD24:0 groups invariably had a higher overall O2 consumption than did the LDN and LD16:8 groups, the differences being caused by reduced O2 consumption in the latter groups during darkness. The LD24:0 groups displayed higher activity than the LDN groups, in which activity was very low at night.

Growth, survival and cannibalism of cod juveniles (Gadus morhua): effects of feed type, starvation and fish size

Abstract The effects of feed type, starvation and fish size on growth, survival and cannibalism in pond-reared cod juveniles (Gadus morhua L.) were investigated in feeding experiments. Ungraded groups of 0.2-g and 8-g cod had 93.5–97% survival in 4 weeks while fed live zooplankton. Average daily growth rate (DGR) was 4.5% and 1.6%, respectively, and this difference is suggested as a possible explanation for the higher losses due to cannibalism in the 0.2-g group. Cannibalism was higher among starved 0.2-g cod than fed 0.2-g cod and the surviving fish from the starved treatments were on average as heavy as the survivors from the fed treatments. Cannibalism among starved 0.2-g cod ceased after addition of live Zooplankton. Large size differences within a cohort may have dramatic effects on the rate of cannibalism. Two large individuals from the 0.2-g group consumed more than 50 siblings under adverse feeding conditions during a 4-week period. In contrast, about 96% of the 8-g fish survived 4 weeks of starvation. Based on the results from the study, an earlier harvest of the juveniles from the rearing ponds is suggested to improve weaning and to reduce losses due to cannibalism.

Growth of juvenile halibut (Hippoglossus hippoglossus) related to temperature, day length and feeding regime

Abstract Experimental studies with farmed juvenile halibut, Hippoglossus hippoglossus , were undertaken to optimize rearing procedures. Extended feeding period and/or day length did not significantly increase growth rates of juvenile halibut of 5 to 20 g wet weight. Growth was strongly affected by temperature, and highest growth rates were obtained at 13°C followed by 10, 16 and 7°C for juveniles of 20 to 90 g. Growth rate declined with size in one of two experiments in this size interval. Q 10 of daily growth rate between 7 and 10°C varied from 2.5 to 3.0. Individual growth always varied highly within the temperature treatments (overall range −0.3 – 3.5%·d −1 ), but significant size rank correlations were maintained during the 12-week experiment. Juvenile halibut grew approximately isometrically from 20 to 90 g. Weight-specific oxygen consumptions of 80 g juveniles averaged 140 and 200 mg O 2 (kg·h) −1 at 10 and 16°C, respectively, and this is comparable to rates measured for other flatfish species.

Effect of somatic and otolith growth rate on stable isotopic composition of early juvenile cod (Gadus morhua L) otoliths

The relative amounts of the stable isotopes of carbon and oxygen in fish otoliths can be used to reveal the environmental history experienced by the fish. This requires that the relative amounts of the isotopes are deposited in equilibrium with the surrounding environment, or that the offset from this equilibrium is known and can be quantified. It is known that carbon isotopes in biogenic carbonates are a mixture of carbon from the seawater and metabolically derived carbon, but the effect of the somatic growth rate of the fish is still unclear. The possible effect of otolith growth rate and fractionation of both carbon and oxygen isotopes are also not established. We carried out a controlled laboratory experiment where we reared cod (Gadus morhua L.) larvae and early juveniles at two temperatures (6 and 10 °C) and generated different growth rates within each temperature by manipulation of prey levels. The otoliths of the resulting fish were analysed for carbon and oxygen isotopes. We found no effect of otolith precipitation rate on fractionation of either carbon or oxygen isotopes. However, there was a depletion of 13C in the otoliths of fish with elevated metabolism. The proportion of metabolically derived carbon in the otoliths was estimated to be 28–32%. Our results suggest that measurements of oxygen isotopes in otoliths can be a reliable tool to estimate ambient temperature since the oxygen isotopes seem to be deposited in the otoliths independently of kinetic and metabolic effects. Fractionation of carbon isotopes in otoliths on the other hand can give valuable insight into metabolism and feeding pattern of fish.

Growth-dependent age estimation in herring (Clupea harengus L.) larvae

A controlled laboratory experiment was carried out to investigate growth-dependent otolith increment formation in herring larvae under constant and variable feeding regimes. Larvae of Norwegian spring-spawning herring were reared at 88C for 46 days using nominal prey densities of 40 (low) or 1200 (high) prey l ˇ1 . Two groups of larvae were offered constant prey levels throughout the experiment (high or low), whereas the prey levels in two other groups were temporarily increased or decreased. All groups were marked twice with alizarin complexone immersion when prey levels were changed (day 18 and day 32 post hatching). Overall survival in the experiment ranged from 36 to 51%, and daily growth in length ranged from 0.02 to 0.4 mm per day. Average daily otolith growth (sagitta) in the low-prey-density group was below 0.2 mm per day, and the apparent increment formation rate was significantly below 1 per day. The high-prey-density group had an increment deposition rate of about 1 per day after the first marking, and these increments averaged 1.1 mm in width. The alizarin marking confirmed that otolith growth responded to increased prey densities and larval growth within a few days, but the response to deteriorating feeding conditions was slower. The apparent increment deposition rate was below 1 per day during the inter-mark period at average otolith growth rates of less than 1 mm per day. In the present study the increment deposition rate was correlated both with larval growth and larval size. The otolith growth pattern confirmed that there was a high correlation between larval size at sampling and previous larval size both within and between groups (rs>0.6 after 4 weeks). The ratios of otolith sizes at different ages within individual larvae could be used to identify groups of larvae and to a large extent also individual larvae originating from different prey regimes. # 2000 Elsevier Science B.V. All rights reserved.

Effects of exposure to extended photoperiods during the first winter on long-term growth and age at first maturity in turbot (Scophthalmus maximus)

The effects of photoperiod on growth of juvenile turbot and the consequences of extended daylength on age at first maturation were investigated. Growth of individually tagged turbot (initial weight 34–44 g, n = 94) was monitored for 18 months. The fish were held under natural photoperiod from hatching (July) until the start of the experiment in December. From December 4, 1991 until May 26, 1992, the fish were reared under constant light (LD24:0, n = 27), 16-h light:8-h darkness (LD16:8, n = 35), or simulated natural photoperiod for 60 °25′N (LDN, n = 32). The fish were then reared together on LDN for 12 months until first maturation during summer 1993 (age 24 months). The fish were held at 16 °C from December 1991 onwards. A growth promoting effect of extended daylength was seen in the LD16:8 and LD24:0 groups, but the effect was not apparent in the LD24:0 group until 6 months after the fish had been transferred to LDN. The final mean weights of the female turbot were 1727 g and 1777 g in the LD16:8 and LD24:0 groups, respectively, whereas final mean weights of males in these groups were 1075 g and 1055 g. In the LDN group the final mean weights for females and males were 1290 g and 909 g, respectively. The results from the present study suggest that exposure to extended photoperiod alters the growth pattern of both maturing fish and immature fish resulting in increased overall growth. Fewer males matured in the LD16:8 (26%) and LD24:0 (17%) groups than in the LDN (56%) group, whereas there were no differences between the experimental groups in the proportion of females that matured (range = 60–63%). It is concluded that extended daylength

Mass Production of Atlantic Cod Juveniles Gadus morhua in a Norwegian Saltwater Pond

Abstract In March-April 1983, 2.5 × 106 yolk-sac Atlantic cod larvae were released in a dammed estuarine pond. One month later, more than half a million metamorphosed. The larvae and metamorphosed juveniles depleted the natural food supply by mid-May, but the fish accepted small pellets containing 30% krill meal dispensed from automatic feeders. From mid-June, young Atlantic cod primarily ate the pellets, supplemented with minor amounts of wild calanoid and harpacticoid copepods. The population declined during summer probably due to cannibalism and predation from birds. No outbreaks of disease were observed, and infestation with parasites (nematodes) was less than 20%. Altogether, 75,000 juvenile Atlantic cod were captured alive from late May to October. By October, about 20,000 15-cm-long juveniles were tagged and released in the Austevoll region in a first attempt to augment the fishery for Atlantic cod.

RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m3 (A) and 4000 m3 (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11-1) compared to in Mesocosm B (>11-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31-1) as compared to Mesocosm B (∼11-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.

Effects of size grading on growth and survival of juvenile turbot at two temperatures

Juvenile turbot were size graded into three size groups (mean initial size): Small (3.4 g), medium (7.0 g) and large (10.5 g), and additional fish were held in ungraded (6.6 g) groups. Subgroups (n = 396) of fish were tagged and individual growth rates and social interactions within different size categories were studied in fish reared at 13 and 19 °C. Size grading of juvenile turbot did not improve growth. Specific growth rates (SGR) were mainly size-related, and no differences in SGR or mortality between the experimental groups at both tem-peratures were found. A higher level of social interactions was indicated amongst medium-sized fish than amongst those in the smallest and largest categories. Excess feeding may have been important factors in reducing aggression, so that the growth of the smallest individuals was not suppressed by the larger individuals in the present study. Grading seems to be an unnecessary operation to improve the growth and survival of juvenile turbot (5- 150 g). However, as it was mainly the smallest individuals in each group that died, grading of very small turbot (2-5 g) can be recommended.