Introducing size limits as a management tool for the recreational line fishery of silver kob, Argyrosomus inodorus (Griffiths and Heemstra), in Namibian waters

Individual silver kob Argyrosomus inodorus (Griffiths and Heemstra) first mature at just over 1 yr of age and the median age at maturity is approximately 1.5 yr. Spawning of silver kob was observed in the southern and central region of the stock's range, but little spawning activity was found in silver kob sampled from the northern region. The spawning period is protracted over 6 months (October–March), which coincides with warmer water temperatures (>15 °C) and the occurrence of large-sized spawning silver kob in the southern and central part of Namibia. Implementing a minimum size limit for the silver kob fishery is not recommended as it will have serious economic implications for the coastal communities. The protection of the spawning areas, Meob Bay and Sandwich, should be continued and a strict bag limit on large-sized silver kob is recommended for the Namibian silver kob recreational fishery.     

Determination of the optimal water temperature for the culture of juvenile dusky kob Argyrosomus japonicus Temminck and Schlegel 1843

The effects of temperature on growth, food-conversion ratio (FCR) and feeding intensity of juvenile dusky kob Argyrosomus japonicus (23.7±2.6 g fish⁻¹) were assessed over the temperature range 17.5–28.5 °C in a 42-day growth trial. Growth increased with increasing temperature up to an optimum after which it declined. Specific growth rates were 2.05% and 1.2% day⁻¹ for the fastest (25.3 °C) and the slowest (17.5 °C) treatments respectively. Food-conversion ratio peaked at a lower temperature than growth. Optimal (0.72 kg kg gain⁻¹) and least efficient (1.40 kg kg gain⁻¹) FCR were found at 21.7 and 17.5 °C respectively. Feeding intensity was linearly related to temperature within the range of 17.5–28.5 °C. These results corresponded to the thermal preference (25–26.4 °C) and natural temperature distribution (12–28 °C) of South African dusky kob. Determination of the temperature range that does not limit growth is a prerequisite to assess the relationship between growth and environmental variables such as light intensity, feeding regime and stocking density. Consequently, experiments to determine the effects of these environmental variables on growth and aquaculture potential of dusky kob should be conducted at 24–26 °C.     

东、黄渤海带鱼的洄游路线

为了研究带鱼的洄游路线,根据我国10余个主要渔业公司1971、1972、1981和1982年带鱼捕捞统计资料,先绘制带鱼鱼群移动影像图,据此进一步研究东、黄渤海近海带鱼的洄游路线图。结果显示,带鱼越冬场、产卵场和索饵场范围广阔,在空间上具有一定的重叠性。带鱼还有分批越冬、产卵和索饵洄游的特性,不同群体越冬、产卵和索饵洄游在时间上也有一定的重叠性。北方带鱼群体的越冬场主要在32°00'～36°00'N,124°00'～126°00'E黄海暖流水域。3—7月北上进入海州湾、乳山湾、海洋岛和渤海各湾产卵场;8—9月在产卵场外围宽阔水域索饵;9—11月游出渤海,绕过成山头南下,沿禁渔线外侧,沿途分别汇合海州湾、乳山湾及海洋岛的越冬鱼群,到了34°00'N后折向东南,分批于12月到达越冬场。南方带鱼群体越冬场主要位于浙江中南部禁渔线外侧,其次是济州岛西南海域。每年3—4月起,南部越冬的带鱼鱼群大致以每2个月1个纬度的速度向东北方向移动;5—7月,产卵洄游的鱼群到达舟山渔场和长江口渔场产卵;7—8月,产卵后的索饵群体北上黄海南部大沙渔场索饵;9月,索饵群体前锋到达35°00'N禁渔线外侧。10月以后,随着冷空气南下,南部群体带鱼向南越冬洄游,并且在10月回到长江口海域。其中小部分群体游向外海越冬场,大部分群体继续南下回到东海中南部近海的越冬场。台湾暖流和黄海暖流的进退和消长是影响带鱼越冬洄游路线空间和时间变化的主要环境因素。     

A spatial model of population dynamics of the early life stages of Japanese sardine, Sardinops melanostictus, off the Pacific coast of Japan

We constructed a numerical model reproducing the transport, survival and individual growth of the early life stages of Japanese sardine, Sardinops melanostictus, off the Pacific coast of Japan during 1978–93. The causes of early life stage mortality, including the influence of the effects of the spatial relationship between the spawning grounds and the Kuroshio on the mortality rate, were investigated. Survival and transport from egg stage to 60 days after spawning were modelled daily in a 1 × 1 degree mesh cell and individual growth in the period was modelled in each region (Kuroshio, Inshore, Offshore and Transition regions). Individual growth and survival from 60 to 180 days after spawning were modelled daily in the Transition region. Environmental data were taken from outside the model system. Our simulation indicates that survival variability in the larval stage (5–25 mm in standard length) is the key factor in determining the year‐class strength. The simulation revealed that strong year classes occurred with good survival in the spawning ground and whilst entrained in the Kuroshio current being transported to the main feeding grounds in the Transition region. The simulation also indicated that survival rates in 1988–93 were low in the Inshore, Kuroshio and Offshore regions, which depressed the year‐class strength during that period.     

Interannual changes in the timing of walleye pollock spawning migration and their impacts on the gillnet fishery in the southwestern Pacific coast of Hokkaido,Donan area,Japan

The southwestern Pacific coast of Hokkaido is the main spawning ground for the Japanese Pacific stock of walleye pollock Theragra chalcogramma. A commercial gillnet fishery targeting spawning adult pollock in this area mainly operates from October to January to coincide with the migration of adult pollock from the feeding ground. Given the results of acoustic surveys, and changes in the proportion of the monthly total catch that was monthly walleye pollock caught by the commercial gillnet fishery, it is thought that the timing of walleye pollock spawning migration to the Donan area varies among years and that the pollock catch of the gillnet fishery clearly reflects changes in pollock abundance in this area. A time series of interannual variability in catch data from 1980 to 2005 suggested that adult pollock migrated and concentrated on their spawning ground later in the 1980s and after 2000 than in the 1990s. Such decadal-scale shifts are presumably caused by climatic changes (e.g., in water temperature) in the Oyashio region. These shifts affect the gillnet fishery through differences in monthly unit prices of pollock and changes in the formation of fishing grounds. These scientific findings can aid the establishment of rules for more efficient walleye pollock resource management under the total allowable catch system.     

Temporal variation of seasonality of egg production and the spawning biomass of Pacific anchovy, Engraulis japonicus, in the southern waters of Korea in 1983–1994

Embryonic mortality, egg production and the spawning stock biomass of Pacific anchovy, Engraulis japonicus , off Southern Korea during 1983–1994, and their biological response to oceanographic features in spring and summer, were analysed. The instantaneous mortality rate (IMR) of embryonic stages decreased in spring and increased in summer, with a range of 0.33–1.23 day^–1 in spring and 0.78–1.69 day^–1 in summer. Egg production in summer was three times that during spring and production was low in the late 1980s. Mean lengths of yolk-sac larvae and adult females were greater in spring than in summer, whereas spawning fraction and spawning stock ratio (spawning biomass:adult biomass) were lower in spring than summer. Estimated mean spawning stock biomass ranged from 141 × 10³ to 380 × 10³ MT in spring and from 221 × 10³ to 557 × 10³ MT in summer. Statistically, the seasonal and long-term trends of embryonic mortality, egg production and spawning stock biomass of Pacific anchovy can be explained largely by spring warming, summer cooling and by less abundant zooplankton in the late 1980s.     

Multi‐decadal variations in spawning ground use in Northeast Arctic haddock (Melanogrammus aeglefinus)

Despite the importance both in an ecological and management context, much uncertainty remains about the underlying factors controlling spawning ground use in marine fish. Here, we investigate how spawning ground use of Northeast Arctic (NEA) haddock (Melanogrammus aeglefinus) has changed over time. By combining data from a Soviet‐Russian egg survey conducted over a 35 year period (1959–1993) with a biophysical drift model, we estimate annual median spawning latitudes of NEA haddock. Furthermore, we test for statistical association between the median latitude of spawning and covariates that are candidates for shaping the spawning distribution of haddock. The results indicate no association with temperature, as measured at the Kola transect, or demographic structure, as measured by the mean weight of spawners. However, we find a positive and significant association between spawning latitude and spawning stock biomass, indicating that density‐dependence may be more important in shaping spawning location than other factors such as temperature and demographic structure. A potential mechanism for the reported effect of spawning stock biomass may act through the distribution at the feeding grounds, which is reported to expand north‐ and eastwards at high population density.     

Differing body size between the autumn and the winter–spring cohorts of neon flying squid (Ommastrephes bartramii) related to the oceanographic regime in the North Pacific: a hypothesis

The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter–spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38–46 cm) and the winter–spring cohorts (ML range: 16–28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different‐sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21–25°C) indicates that the spawning ground occurs farther north during autumn (28–34°N) than winter–spring (20–28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food‐rich nursery ground in winter. The winter–spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter–spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food‐rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter–spring cohort in summer.     

Migration of kawakawa Euthynnus affinis in the waters near Taiwan

ABSTRACT:   Kawakawa Euthynnus affinis is the most abundant species among the small tuna in the coastal waters of Taiwan. The migration pattern of kawakawa to Taiwan was determined in the present study based on the species' feeding habits and reproductive biology and on temporal variation of catch for kawakawa and its prey fishes. Schools of this species successively migrate to Taiwan from September to May, and then move away in June. They travel to the waters off Taiwan with the warm Kuroshio current, migrate toward coastal waters for feeding and move away for spawning. It is suggested in the present study that the kawakawa spawning ground is in the waters of the northern Philippines. The migration of kawakawa exhibits size-segregating behavior. Schools of immature young fish first arrive in the coastal waters around Taiwan, followed by the adults.     

Spawning, early development and first feeding in the gobiid fish Priolepis nocturna

The reproductive behavior, embryonic development and early larvae of Priolepis nocturna are described. Three pairs of P. nocturna began spawning 41 days after acquisition and maintained a 5–10 day spawning cycle lasting beyond several months. Spawning was initiated by the female who signaled her readiness to spawn by displaying to the male. Egg clutch size averaged 1578 ± 51.23 eggs and ranged from 268 to 3121. Egg length averaged 0.82 ± 0.01 mm total length (TL) and ranged from 0.75 to 0.90 mm. Egg width averaged 0.51 ± 0.51 mm total width (TW) and ranged from 0.49 to 0.52 mm. Fertilized eggs were ovoid in shape and attached to the ceiling of provided shelters via adhesive filaments at the proximal end. Hatching rates averaged 97.3 ± 0.51% and ranged from 91.9 to 99.8%. Larvae measuring 1.89 ± 0.04 mm TL hatched 121 ± 0.5 h post fertilization and did not rotate position prior to hatching. Skeletal elements of the chondrocranium were simplistic and dominated by the hyoid, hyomandibulosymplectic cartilage, ethmoid and Meckel's cartilage in first feeding larvae. No elements were added to the cranial architecture by 5 days post hatch (DPH) when larvae measured 2.05 ± 0.04 mm TL. First feeding larvae consumed only dinoflagellates and tintinnids suggesting that feeding was constrained by a poorly developed feeding mechanism. Embryology and larval development are described to 5 DPH.     

Migration and spawning behavior of the greater amberjack Seriola dumerili in eastern Taiwan

Greater amberjack (Seriola dumerili) is an important fishery resource with a circumglobal distribution from tropical to temperate waters. Here, we investigated the spawning migration and habitat utilization of S. dumerili in the East China Sea (ECS). Archival tags were attached to 22 adult fish to examine their horizontal and vertical movements and estimate the spawning ground. S. dumerili were captured and released in the coastal waters of eastern Taiwan on November of 2016 and 2017. Information from seven pop-up satellite archival tags and seven depth–temperature recorders was recovered. Almost all of the fish stayed in the Taiwanese exclusive economic zone. Most individuals moved from released site to the southern edge of the ECS and showed behavior associated with the topographic features in winter (November to December). These phenomena may be related to foraging and be driven by oceanographic features such as the seasonal monsoon and the Kuroshio. The fish then migrated to the south offshore area of Taiwan in January and February. During their southward migration, the fish experienced a slowly elevated water temperature regime (SETR), which is one of the environmental factors that induce final oocyte maturation. In the spawning season (February to April), tagged females exhibited continuous diel vertical movements (DVMs) after experiencing the SETR. These continuous DVMs were observed over a wide geographic range from north to south in the Kuroshio off eastern Taiwan. Our study demonstrated that the putative spawning ground of S. dumerili must extend further in a north–south direction than predicted in a previous study.     

Distribution, larval stage duration and growth of the sea lamprey ammocoetes, Petromyzon marinus L., in a highly modified river basin

Abstract  – Larval stage duration of the sea lamprey, Petromyzon marinus , in the River Mondego is estimated to last for 4 years. The number of annuli provides reliable age estimates when compared with length–frequency distributions analysis. The growth rate of the sea lamprey ammocoetes displays strong seasonal patterns, and reaches its highest value during the first 2 years of larval stage. About 69% of the length increment between hatching and metamorphosis is attained at the end of the second year. There is a longitudinal gradient associated with ammocoete distribution along the river. Relative abundance of ammocoetes decreases downstream from the Açude-Ponte dam, the first obstruction encountered by the adult sea lampreys in their upstream spawning migration along the River Mondego.     

Variations in spawning ground area and egg density of the Japanese sardine in Pacific coastal and oceanic waters

The spawning ground of the Japanese sardine, Sardinops melanostictus (Schlegel), was distributed over the oceanic waters as well as the coastal waters along the Pacific coasts of western and eastern Japan during 1978–1992. The area of the spawning ground in the coastal waters on the continental shelf has ranged from 95 000 km² in 1992 to 143 000 km² in 1988, constituting 44–77% of the total area of the spawning ground. The area of the coastal spawning ground was relatively constant in spite of the large fluctuations in egg abundance, i.e. size of the spawning population, from 88 trillion (1987) to 668 trillion (1989) in the waters. Spawning adults seemed to extend over the coastal waters irrespective of the size of the spawning population. In contrast to the coastal waters, the spawning area in the oceanic waters offshore of the continental shelf increased from 31 000 km² in 1978 to 183 000 km² in 1988 and then shrank to 83 000 km² in 1992, as a function of the spawning population size. The egg distribution density in the coastal waters stayed less than 6000 m⁻² mo⁻¹, but it reached as high as 27 400 m⁻² mo⁻¹ in the expanded spawning ground in the oceanic waters. The oceanic waters seemed to function as a reserve spawning ground for the sardine in years of extremely high spawning population.     

Staged spawning migration in Icelandic capelin (Mallotus villosus): effects of temperature,stock size and maturity

Capelin (Mallotus villosus) is the largest commercial fish stock in Icelandic waters and also an important forage fish. Every winter pre‐spawning capelin migrate 500–1000 km from their offshore (>200 m bottom depth) northern feeding areas (67–71°N) to inshore (<200 m bottom depth) southern spawning areas (63–65°N). The major migration route is east of Iceland. The route consists of both offshore and inshore phases. The migration begins offshore as capelin skirt the shelf edge north of 65°N, then abruptly veer inshore between latitudes 64° and 65°N. Hydro‐acoustic data from 1992–2007 demonstrated that the timing of the offshore phase migration varied by as much as 1 month, from 22 December to 21 January. A combination of larger spawning stock and colder feeding ground temperatures (August–December) resulted in earlier offshore migration. The timing of the inshore migration phase was not dependent on the offshore migration timing, and never began prior to the first week of February. Many cohorts arrived at latitudes 64–65°N in early January but staged offshore at latitudes 63.8–65.8°N until early February. The longest observed delay in the staging area was 5 weeks. Timing of the inshore migration was controlled by gonad maturity, with migration beginning when roe content attained 12–14%. Staging limited the time capelin spent on the continental shelf before spawning to 3 weeks. We suggest that offshore staging evolved to minimize temporal overlap with predatory gadoids, especially Atlantic cod (Gadus morhua).     

Alternative use of food resources causes intra-cohort variation in the size distribution of young-of-the-year perch (Perca fluviatilis)

Abstract –  Body sizes of young-of-the-year (YOY) perch ( Perca fluviatilis ) at the end of their first summer are extremely variable and range in different studies between 4 and 15 cm. To analyse whether size divergences in YOY perch may be attributed to alternative use of food resources, adult perch were stocked into two previously fishless ponds and growth, size distribution and food intake of the YOY perch were recorded. In addition to perch, adult bream ( Abramis brama ) were introduced to produce juvenile bream that could serve as a food resource for YOY perch. The body sizes of YOY perch at the end of the experiment ranged from 32 to 168 mm with a bimodal size distribution. The combination of stomach content analyses and stable isotope signatures revealed that the small size cohort were planctivorous/benthivorous while the large size cohort was piscivorous/cannibalistic. Results implicated that different feeding behaviour contributed to the size divergences in YOY perch and that the extreme growth of the large size cohort was induced by piscivory.     

Migration and escapement of silver eel males,Anguilla anguilla,from a southwestern European river

The catadromous life cycle of the European eel encompasses a spawning migration to the Sargasso Sea. Prior to their migration, eels develop a silver coloration being referred to as “silver eels.” Due to the dramatic European eel recruitment decline, it is crucial to quantify silver eel escapement to evaluate the success of measures taken under Eel Management Plans (Regulation EC (1100/2007)). Thus, the percentage of silver eels escaping from the River Mondego (Portugal) was estimated during two consecutive spawning seasons (2014–2016) and their migratory behaviour was studied. Thirty-six male silver eels were tagged with acoustic transmitters and their downstream migration was tracked using automatic receivers deployed at four locations along the river course, from the first impassable obstacle up to the river mouth (67 km). Among the tagged eels, 15 (42%) escaped to the sea. In both years, individuals displayed faster migration speed in the downstream reaches of the river. BRT (“Boosted Regression Trees”) models were used to explain downstream migration and escapement of silver eels. The analyses revealed that the downstream migration is triggered by several environmental variables with water conductivity as the most influential predictor. Escapement of eels from the estuary occurred a few hours after their arrival and was primarily explained by the offshore wind intensity, which promotes a current in the coast that may favour their transport to the open sea. These findings contribute to a better understanding of silver eel migration in southern latitudes and highlight and reinforce the need to monitor spawner escapement.     

Reproductive biology of <Emphasis Type="Italic">Sillago aeolus</Emphasis> in Okinawa Island,Japan

ABSTRACT:   A total of 414 fish (female, 59.3–275.0 mm and male, 61.0–220.0 mm standard length) were collected from December 2000 to March 2002 around Okinawa Island, Japan, for the determination of sex, spawning season, maturity and fecundity. Monthly changes in the gonadosomatic index (GSI) exhibited similar trends for both sexes and the mean GSI maintained a high level between February and May. Furthermore, the percentage frequency of hydrated oocytes and the spermatozoic activity suggested that main spawning occurred between February and May. Some hydrated oocytes were found throughout the year, with a GSI value greater than 1.0, suggesting that sporadic spawning occurred. Length at first maturity was determined for females and males as 120 and 113 mm standard length, respectively. Almost 50% and 100% males were matured at the end of their 0 and 1 year of age group, respectively. In contrast, no females were found to be mature at 0+ age group, and almost 60% of females were mature at 1+ age group. At ages over 2 years, all males and females were found to be mature. Batch fecundity (BF) of 33 females was related to standard length, and the relationship between standard length and BF was expressed by the exponential equation: BF = 269.5e^0.020954SL.     

Functional role of environmental stimuli for the spawning migration in Danube nase Chondrostoma nasus (L.)

Abstract –  In spring 2004 and 2005, an online monitoring by horizontal split-beam hydroacoustics complemented by trap net catches and electro fishing was conducted to investigate the spawning migration of nase Chondrostoma nasus (L.) into the Fischa River, a tributary of the Danube River east of Vienna, Austria. Upstream-moving adult nase could be counted hydroacoustically based on their dominance (93%) of fish in the range 45–55 cm total length. Significant correlations were observed between the number of migrants and three environmental factors (water temperature, water level and turbidity), with special focus on their fluctuations. Thresholds of water temperature, water level and turbidity were revealed as general environmental cues on a seasonal scale. On a fine temporal scale, the fluctuations of the environmental factors over days, especially in the main river, represent relevant stimuli; they initiate and control the fine-scaled temporal pattern as well as the intensity of the spawning migration. In addition, a combination of increasing water temperatures and decreasing water levels, corresponding to the period after a flood peak, offers high predictability of favourable spawning conditions at the spawning place and acts as a stimulus for the right timing for upstream migration in Danube nase.     

Using coupled fish behavior–hydrodynamic model to investigate spawning migration of Japanese anchovy,Engraulis japonicus,from the East China Sea to Taiwan

Adult Japanese anchovies (Engraulis japonicus) migrate from the East China Sea to the coastal region of Taiwan to spawn around late winter and early spring and, later, their larvae constitute important fisheries in Taiwan. However, their migration route and its mechanism remain unclear. To investigate their spawning migration, we used a coupled fish behavior–hydrodynamic modeling approach. The physical field is simulated by the Pacific Ocean adaptation of the TaIwan Multi‐scale Community Ocean Model (TIMCOM) and the fish migration by Lagrangian tracer tracking with the aid of approximation of fish swimming behavior. We investigated three fish behavioral scenarios: (i) passive tracking of the current, (ii) swimming along with the current, and (iii) swimming along with the current and then changing to swimming toward the optimal spawning temperature. The comparison with and without Changjiang discharge is used to investigate the impacts of discharge reduction due to the Three Gorges Dam. Our results suggest that spawning migration of Japanese anchovy from the East China Sea to Taiwan may be aided by the China Coastal Current and that adult anchovies cannot reach the spawning site by passive advection alone. Thus, the swimming behavior of anchovies is crucial during the spawning migration, as it provides extra velocity and the orientation to the favorable spawning grounds. In addition, the adult anchovy is unlikely to reach the coastal area of Taiwan without Changjiang discharge. Our findings indicate that a coupled fish behavior–hydrodynamic model can help understand the influences of physical environment on the migration of Japanese anchovies.