Olav Sigurd Kjesbu

Total lipid energy as a proxy for total egg production by fish stocks

The indeterminate relationship between the total biomass of mature fish (spawner biomass) and the number of offspring produced (recruitment) has puzzled population dynamicists and impeded fisheries management. The relationship assumes that spawner biomass (in tonnes) is proportional to the total number of eggs produced (TEP) by the stock, an assumption under increasing challenge. Most stocks require proxies for TEP because contemporary and/or historical fecundity data are lacking. Here we show a positive association between recruitment and the liver weights of spawners in the Barents Sea cod stock which suggests that recruitment is constrained by the amount of lipid energy stored in the liver. This stimulated our interest in estimating total lipid energy (TLE; in kilojoules) for mature females in the stock. We examined the suitability of TLE as a proxy through correlation and simulation analyses. The results indicate that TLE is proportional to TEP and exhibits a similar response to varying food abundance. Replacing spawner biomass with more accurate measures of reproductive potential is essential to developing a rational basis for stock conservation. Correctly specifying the first-order maternal effect on TEP is a prerequisite to detecting environmental and ecological effects on recruitment.

Effects of adult age and size structure on reproductive output in marine fishes

In recent decades, levels of exploitation exerted on many fish stocks* have been extreme and the fishing gear frequently employed has removed large members of stocks (Policansky, 1993; Garrod and Schumacher, 1994). As a consequence, individuals in overexploited stocks are often unable to attain the ages and body sizes reached by individuals during periods when fishing pressure was considerably lower (Leaman, 1991; Anon., 1995a). Among other ramifications, the depletion of large fish may seriously lower a stock’s egg production, but stock reproductive potential is further diminished if poorer gamete quality is exhibited by younger compared with older members. Studies on several iteroparous marine fishes and a variety of animals have shown that brood characteristics will change with each successive breeding season and there is evidence for an increase in brood quality with parent reproductive experience (Parker and Begon, 1986; Sargent et al., 1987).

Growth, gonadal development and spawning time of Atlantic cod (Gadus morhua) reared under different photoperiods

Abstract Individually tagged Atlantic cod ( Gadus morhua L . ) (mean body weight 459±6 g) were reared in 20-m 3 tanks supplied with running sea water. During the first year the fish were exposed to one of four photoperiod regimes: Natural light (LDN), continuous light from June 23, 1993 (LL), natural light from June 23, 1993 and continuous light from December 22, 1993 (LDN/LL) or continuous light from June 23, 1993 and natural light from December 22, 1993 (LL/LDN). In July 1994, the LDN and LDN/LL groups were terminated. The LL and LL/LDN groups were reared on until May 1995. From July 1994 to the termination of the experiment in May 1995, the LL group was reared under continuous light and the LL/LDN group was reared under natural photoperiod. Cod reared under natural photoperiod spawned in the period between January and April. Photoperiod manipulation changed the incidence of sexual maturation, spawning time, fecundity and egg size. Cod that were transferred from natural photoperiod to continuous light in December spawned earlier, having lower fecundity and smaller eggs than the cod reared under natural photoperiod. Oocytes of females reared under continuous light were arrested in the cortical alveoli stage, and even in their second year on continuous light very few females matured. When transferred to natural photoperiod in December, females ovulated within 4–5 months, approximately 3 months delayed compared to the natural photoperiod group. These cod had a higher fecundity and smaller eggs than natural photoperiod cod. When reared on for another year under natural photoperiod these cod spawned again the next year at their normal spawning time, e.g. 9 months after their first spawning. The pattern of sexual maturation influenced the somatic growth pattern. Differences in growth were explained by differences in the timing of the spawning and the relative spawning investment. At an age of 26 months, the weight of the cod reared under natural photoperiod and continuous light were 1.5 and 2.5 kg, respectively. From this, it is concluded that a reduction in daylength is a vital environmental signal regulating the maturation and spawning of cod, and that sexual maturation may be arrested or considerably delayed in its absence.

Oocyte growth and fecundity regulation by atresia of Atlantic herring (Clupea harengus) in relation to body condition throughout the maturation cycle

Oocyte growth, fecundity regulation by resorption of vitellogenic oocytes (atresia), and condition effects on fecundity for repeat spawners (z32 cm in total length (TL)) of Norwegian spring-spawning (NSS) herring, Clupea harengus, were examined using samples collected periodically from July 1998 to February/March 1999. This period almost covered the maturation cycle of the fish, i.e., 67% (30/45) of the examined fish had started vitellogenesis as early as in July and 18% (7/40) showed hydrated oocytes in February/March. Oocyte diameter increased linearly over time. Average fecundity of 34 cm TL fish decreased by about 56% from 113000 in July to 49200 in February/March. Both prevalence of atresia (portion of fish with atresia) and average relative intensity of atresia (prevalence multiplied by geometric mean of relative intensity of atresia among only fish with atresia) were highest in October and November, i.e., following the summer feeding season when fish started to rely on accumulated body reserves. Estimated duration of atresia was 4.5, 6.8, 6.1 and 7.2 d for July–October, October–November, November–January and January–February/March, respectively. Atresia seemed to be limited to oocytes smaller than 1100 Am, which had lipid and solids (protein, ash and carbohydrates) contents that were only half of the values observed for fully matured oocytes (1400–1550 Am). Both the timing of intensive resorption and size of atretic oocytes seemed to optimise fecundity given available energetic reserves. There appeared a highly significant, positive correlation between ovary dry weight, a proxy of reproductive investment, and muscle dry weight condition factor (MDCF; 100 � muscle dry weight/TL 3 ) in the later maturation cycle. Relative fecundity also showed a significant, positive correlation with MDCF in February/March. In conclusion, this study demonstrates important energetic and cellular mechanisms for regulation of reproductive investment in NSS herring females, a long-lived, temperate capital breeder. D 2003 Elsevier Science B.V. All rights reserved.

Effects of periodic starvation on reproductive investment in first-time spawning Atlantic cod (Gadus morhua L.)

Prior to the onset of sexual maturation 1-year-old farmed Atlantic cod were either subjected to periods of short-term starvation ranging from 3 to 9 weeks (3 alternating periods of 3 weeks starvation separated by 1 week of feeding) in duration or were held on full rations from July to November. At spawning the fish that had been starved for 9 weeks weighed 60% of those held on full rations. The fish held on full rations had significantly higher fecundities than the fish that had been starved for 9 weeks, but these differences were related to differences in body size; relative fecundities were similar for fish in both groups. Reduced growth and liver sizes in feed-restricted groups did not result in lower proportions of maturing fish. Thus, the imposition of short periods of starvation did not solve the problem of early maturation in cod culture. Throughout a combined starvation and spawning period from January until May, females lost more of initial total, somatic and liver weight than males. The analysis showed that, in total, females invested more in reproduction than males.

Gonadal development and associated changes in liver size and sexual steroids during the reproductive cycle of captive male and female Atlantic cod (Gadus morhua L.)

Gametogenesis in female and male Atlantic cod (Gadus morhua L.) was investigated by sampling blood plasma and gonadal tissue from 19 to 33-month-old fish. The reproductive cycles of both female and male Atlantic cod are characterized by distinct annual variations in gonadal size and developmental stage and these are associated with changes in sex steroids and liver size. I(H) did not change during early gonadal development, but both spent females and males had lower I(H) than late maturing females and spermiating males, respectively. In females I(G) was correlated to plasma E2 levels and they were highest in spawning females. The lowest levels during the reproductive cycle were observed in spent females. Plasma T levels were low throughout ovarian development, and were at a minimum in spent females. 11-ketotestosterone in plasma of males increased rapidly during spermiation, while T increased at earlier testicular stages and reached maximum during spermiation. High plasma levels of steroids in male and female cod during spawning serve to promote further development and growth of less advanced stages of germ cells.

Predictions of realised fecundity and spawning time in Norwegian spring-spawning herring (Clupea harengus)

Maturing Norwegian spring-spawning (NSS) herring, Clupea harengus, were collected for reproductive analyses along the Norwegian coast prior to the spawning seasons of 1997–2000. Over this time period there was a marked change in weight (W) at length (TL) with 1998 showing extremely low values and 2000 high values in a historical perspective. Potential fecundity, amounting to about 20000–100000 developing (vitellogenic) oocytes per fish and positively related to fish size, increased significantly with fish condition. Relative somatic potential fecundity (RFP, number of oocytes per g ovary-free body weight) in NSS herring was found to vary by 35–55% between years. Unexpectedly, females in 2000 showed low RFP-values, possibly due to negative feedback from previous reproductive investments at low condition. A clear threshold value for Fulton’s condition factor, K (K=100 � W/TL 3 ), of 0.65–0.70 existed below which there was considerable atresia (resorption of vitellogenic oocytes). Thus, these components of the spawning stock, amounting to 1–46% in the period 1980–1999, obviously contributed relatively little to the total egg production. This was confirmed by low ovary weights and examples of delayed oocyte development in these individuals. An up-to-date atresia model is presented. The established oocyte growth curve, and to a lesser degree the assumed atretic oocytic turnover rate, was critical for the estimation of realised fecundity (number of eggs spawned). Modelled realised fecundity was significantly below observed potential fecundity. Females that had migrated the shortest distance from the over-wintering area, Vestfjorden, northern Norway, were in the poorest condition, had the least developed oocytes and the lowest potential and realised fecundities. In agreement with previously published studies on temporal and spatial changes in gonad weights, those females reaching the main spawning grounds in the south–western part of the coast (More) were the most successful ones in terms of egg production. Likewise, present results on oocyte diameter confirmed that repeat spawners spawn first and recruit spawners second. Our histological analyses on oocyte microstructure provided further evidence that oocyte size is a precise and accurate maturation criterion in herring. The methodological examinations also showed that the level of atresia as well as potential fecundity from oocyte and ovarian size can be estimated by the binocular microscope. This study shows that there is a large range in size- and condition-specific egg production in NSS herring, which should be taken into account in further recruitment studies. D 2002 Elsevier Science B.V. All rights reserved.

Thermal dynamics of ovarian maturation in Atlantic cod (Gadus Morhua)

The timing and success of spawning in marine fish are of fundamental importance to population persistence and distribution and, for commercial species, sustainability. Their physiological processes of reproduction are regulated, in part, by water temperature, and therefore changes in marine climate may have dramatic effects on spawning performance. Using adult Atlantic cod (Gadus morhua) as a case study, we examined the links between water temperature, body size, vitellogenesis, and spawning time by conducting extensive laboratory and field studies. Our experiments documented that vitellogenesis generally starts at autumnal equinox and that oocyte growth and investment are greater in cod held at warmer temperatures. Furthermore, spawning occurred earlier when oocyte growth was more rapid. Large females spawned earlier than smaller females at warmer temperatures, but this effect vanished at colder temperatures. The experimental results were confirmed by measurements of oocyte growth collected from wild-cau...

Synergies between climate and management for Atlantic cod fisheries at high latitudes

Significance Currently many exploited fish populations, including many of the Atlantic cod stocks, are at historically low levels with widespread concern about whether contemporary management is capable of facilitating population recovery. In contrast, the spawning stock biomass of Barents Sea cod is now at an historic high. Here we demonstrate that successful management actions interacting synergistically with prevailing climate caused this increase. Warming of water masses in the Barents Sea over the last decade positively reinforced management actions. A unique and possibly generic mechanism of climate affecting marine animals at high latitudes, especially when at the polar extreme of their distribution, is identified: adjustment of the suitable feeding area. This adjustment is linked closely to community dynamics and increased stock productivity. The widespread depletion of commercially exploited marine living resources is often seen as a general failure of management and results in criticism of contemporary management procedures. When populations show dramatic and positive changes in population size, this invariably leads to questions about whether favorable climatic conditions or good management (or both) were responsible. The Barents Sea cod (Gadus morhua) stock has recently increased markedly and the spawning stock biomass is now at an unprecedented high. We identify the crucial social and environmental factors that made this unique growth possible. The relationship between vital rates of Barents Sea cod stock productivity (recruitment, growth, and mortality) and environment is investigated, followed by simulations of population size under different management scenarios. We show that the recent sustained reduction in fishing mortality, facilitated by the implementation of a “harvest control rule,” was essential to the increase in population size. Simulations show that a drastic reduction in fishing mortality has resulted in a doubling of the total population biomass compared with that expected under the former management regime. However, management alone was not solely responsible. We document that prevailing climate, operating through several mechanistic links, positively reinforced management actions. Heightened temperature resulted in an increase in the extent of the suitable feeding area for Barents Sea cod, likely offering a release from density-dependent effects (for example, food competition and cannibalism) through prolonged overlap with prey and improved adult stock productivity. Management and climate may thus interact to give a positive outlook for exploited high-latitude marine resources.

Frequent skipped spawning in the world’s largest cod population

Life-history theory suggests that animals may skip reproductive events after initial maturation to maximize lifetime fitness. In iteroparous teleosts, verifying past spawning history is particularly difficult; the degree of skipped spawning at the population level therefore remains unknown. We unequivocally show frequent skipped spawning in Northeast Arctic cod (NEAC) in a massive field and laboratory effort from 2006 to 2008. This was verified by postovulatory follicles in temporarily arrested ovaries close to the putative spawning period. At the population level, “skippers” were estimated to be approximately equally abundant as spawning females in 2008, constituting ∼24% of the females 60–100 cm. These females never truly started vitellogenesis and principally remained on the feeding grounds when spawners migrated southward, avoiding any migration costs. The proximate cause of skipping seems to be insufficient energy to initiate oocyte development, indicating that skipped spawning may partly be a density-dependent response important in population regulation. Our data also indicate more skipping among smaller females and potential tradeoffs between current and future reproductive effort. We propose that skipped spawning is an integral life-history component for NEAC, likely varying annually, and it could therefore be an underlying factor causing some of the currently unexplained large NEAC recruitment variation. The same may hold for other teleosts.