Changes in reproductive patterns in relation to decline in stock abundance of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay

ABSTRACT:   We investigated changes in the reproductive patterns of mantis shrimp Oratosquilla oratoria concurrent with stock-abundance decline in Tokyo Bay, Japan. Stock abundance was high in the mid to late 1980s but decreased abruptly in the early 1990s. The yearly change in annual mean larval abundance was similar to that of stock abundance. Mantis shrimp in the bay have two spawning seasons, an early season (May–June) for ≥1-year-old individuals and a late season (July–September) for 0–1-year-old individuals. This general reproductive pattern does not differ among different stock-abundance levels. However, the monthly pattern in larval abundance has changed with stock-abundance decline; larval abundance from the early spawning season was highest in the high-stock-abundance period, and it decreased significantly in the low-stock-abundance period, probably as a result of decreased spawning-stock abundance of large female mantis shrimp ≥1-year-old. Correlation analysis on the egg production index and larval abundance suggested that during this low-stock-abundance period the population is supported mostly by late-hatched larvae spawned by small, 0–1-year-old female mantis shrimp. Considering the reproductive pattern and the present status of the fishery, the stock of small female mantis shrimp should be conserved to enhance reproduction of the population for stock recovery.     

Relationship between body length, processed-meat length and seasonal change in net processed-meat yield of Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay

ABSTRACT:   There is concern regarding a possible decrease in the minimum exploitable size of the mantis shrimp Oratosquilla oratoria in Tokyo Bay, coincident with a decline in stock size. To assess this problem, the condition factor, the relationship between body length and processed-meat length, and the estimated body length for each market size category were investigated and compared monthly. The condition factor was lowest in spring and peaked in winter. The difference between body length and meat length was higher in summer and winter than in spring and autumn, suggesting that a high condition factor does not result in an increased net yield of processed meat. The annual mean minimum exploitable size in the present low-stock-size condition did not differ from the minimum exploitable size measured during a high-stock-size period (11-cm body length). However, the minimum exploitable size changed seasonally, which coincided with seasonal change in the net processed-meat yield. Exploitation during the season when net meat yield is high would result in increase of the large-sized meat products of high price.     

Age estimation of the wild population of Japanese mantis shrimp <Emphasis Type="Italic">Oratosquilla oratoria</Emphasis> (Crustacea: Stomatopoda) in Tokyo Bay,Japan, using lipofuscin as an age marker

ABSTRACT:   In this study, the age composition of the Japanese mantis shrimp, Oratosquilla oratoria , in Tokyo Bay, central Japan, was investigated using lipofuscin, an autofluorescent pigment, as an age marker. Lipofuscin in a histological section of protocerebral bridge cell mass (PBCM) in the brain was identified by confocal microscopy, and its concentration was quantified by image analysis. Modal analysis of the lipofuscin concentration showed four regularly-spaced modes that could each be regarded as a distinct age group. This implied a constant lipofuscin accumulation in PBCM at a 6.5 × 10⁻²% volume fraction per year; it also implied the existence of individuals that are at least 4 years old. The lipofuscin concentration was found to be a more appropriate index than body length for estimating the age of O. oratoria , because the modal analysis on the body–length histogram failed to detect apparent age groups. This was probably a result of the declining growth rate with age and the individual variations in growth through molting leading to considerable overlap in the size between different age groups. The lipofuscin analysis suggested that fast-growing individuals in each cohort are recruited to the fishery, and most individuals attain the exploitable size between 2 and 3 years of age.     

渤海口虾蛄仔虾季节性分布及其早期补充模式

仔虾是口虾蛄(Oratosquilla oratoria)早期生活史的重要阶段,其转为底栖生活所需的环境条件及我国近海口虾蛄早期补充模式,仍缺乏了解。基于2017年春季(3―6月)和夏秋季(8―11月) 8个航次的渤海渔业资源底拖网调查数据,分析了各月份口虾蛄仔虾(<60 mm)的分布、出现概率和样本量等,并利用广义加性模型筛选出影响仔虾出现概率的主要环境因子。通过文献综述,回顾了我国近海口虾蛄产卵期、胚胎和幼体发育、仔虾生长特性等繁殖特征和早期生长发育历程,结合仔虾季节性分布,阐释渤海口虾蛄群体的早期补充模式。结果发现,春季,仔虾在4―5月出现频次较高,其分布区随水温升高逐步向北扩散或偏移,显著影响其出现概率的环境因子为底层温度、底质平均粒径和粒径偏度;夏秋季,仔虾在9―10月分布广泛,11月趋向斑块化分布、出现频次降低,显著影响因子为底层温度、底层盐度和底质含沙百分比。渤海口虾蛄群体呈现多季节补充模式。其产卵期长达5个月,5―7月产出的受精卵预计在7―9月发育成Ⅰ期仔虾,在9―11月长成全长50 mm左右的仔虾;8―9月产出的受精卵,预计在10―11月左右发育成Ⅰ期仔虾,12月至次年3月受低温限制停止生长,在4―5月水温回升时逐步长成50 mm左右的仔虾。     

Interannual variation in quantitative relationships among egg production and densities of larvae and juveniles of the Japanese mantis shrimp <Emphasis Type="Italic">Oratosquilla oratoria</Emphasis> in Tokyo Bay,Japan

To explore which lifestages affect the stock size of young-of-the-year mantis shrimp Oratosquilla oratoria in Tokyo Bay, Japan, we investigated interannual variations in the quantitative relationships among egg production, larval density, and juvenile density. We collected adult females, larvae, and juveniles during monthly field surveys from 2004 to 2007. The interannual trend for the juvenile density index differed from those for egg production and larval density; although indices of both egg production and larval density were high in 2004 and 2007, the juvenile density index was high only in 2007, suggesting high mortality during the pelagic larval stage or the early phase of the postsettlement juvenile stage in 2004. We found that larval settlement started at the end of August and peaked in October, although larvae from the early spawning season (May–June) should have settled in August or earlier. Juveniles were found throughout the bay except in areas where bottom hypoxia occurred, suggesting that hypoxia restricts the spatial distribution of juveniles. Our results suggest that mortality during the early life history fluctuates among years, probably because of changes in environmental conditions in the bay, resulting in interannual variation in the stock size of young-of-the-year juvenile O. oratoria.     

Long-term changes in the assemblage of demersal fishes and invertebrates in relation to environmental variations in Tokyo Bay, Japan

Abstract Long-term changes in the demersal assemblage, including both fishes and invertebrates, in Tokyo Bay, Japan, were examined between 1977 and 1995. A total of 255 species were collected, of which 113 were regarded as representative in terms of occurrence. Multivariate analyses of the representative species showed that fluctuations in the catch were related to three periods that reflected changes in relative abundance and species composition. In particular, mantis shrimp, Oratosquilla oratoria (de Haan), whipfin dragonet, Repomucenus valenciennei (Temminck & Schlegel), and marbled sole, Pleuronectes yokohamae Günther, showed high dominance in the mid-1980s. Discriminant analysis suggested that the demersal assemblage changed synchronously with environmental conditions. Causes of the temporal changes in the demersal assemblage in relation to variations in environmental factors are discussed.     

Maturation and spawning of the small penaeid shrimp Metapenaeopsis dalei in Tosa Bay, Pacific coast of southern Japan

SUMMARY: Seasonal changes in the percentage of mature female Metapenaeopsis dalei were studied histologically, and the annual cycle of maturation and spawning of this species was discussed. Specimens were collected at night by a beam trawler in Tosa Bay (33°N, 133°E) during the period from April 1995 to March 1996. All individuals had a gonado somatic index [ GSI = 100 (gonado weight/body weight)] of higher than 8, and 66.7% of those individuals with a GSI of 6–8 had oocytes in the late phase of the prematuration stage after germinal vesicle shrinking and migrating to the margin of the oocyte or had oocytes in the maturation stage. Such individuals with developed oocytes appeared in April, June, July, August, October, November and March. Individuals with a GSI above 6 appeared throughout the year, except during September and February. Individuals with ovulated oocytes were sampled in March when the water temperature near the bottom depths was lowest (15.4°C) during the year, being similar to February (15.1°C). These facts indicate that maturation occurs all year-round and suggests that spawning in the Metapenaeopsis dalei population in Tosa Bay occurs all year-round. This is the first report of year-round maturation in penaeid shrimps in Japanese waters.     

Reproductive biology of ridgeback shrimp Solenocera choprai (Decapoda, Penaeoidea, Solenoceridae) off Mangalore coast, south India

Abstract:   The ridgeback shrimp Solenocera choprai has emerged as one of the important commercial species in 60–100 m depth zone off Karnataka coast, India, since 1993. Studies were conducted on S. choprai to understand its reproductive biology which can help in formulating policies for management of the species. The diagnostic features of reproductive morphology of the species were studied and illustrated in detail. Based on the coloration and size of the ovary and ova diameter variations, five stages of maturity in female S. choprai were identified. The mature ova are fully yolked with diameters between 0.24 and 0.35 mm. Size at maturity (50%) has been estimated at 54.5 and 66.5 mm total length (TL) in males and females, respectively. The gonadosomatic index (GSI) showed an increasing trend according to the advancement of maturity of the ovary, and monthly mean GSI showed a positive correlation with monthly percentage of spawners. Solenocera choprai was found to be a continuous breeder with two peaks: a major peak in November and another extended peak during January–February. The estimated numbers of ova in the mature ovary were 38 532 and 133 689 in shrimps measuring 80 and 110 mm TL, respectively. A sigmoidal increase in the female-to-male sex ratio with increase in size was noticed in the species. A χ ²-test showed that the annual distribution of females and males is not significantly different from a 1:1 ratio at the 1% level.     

Reproductive cycle of yellow sea bream <Emphasis Type="Italic">Dentex tumifrons</Emphasis> in Wakasa Bay,the Sea of Japan off central Honshu

ABSTRACT:   To determine the spawning season and reproductive cycle of the yellow sea bream, Dentex tumifrons , in Wakasa Bay, in the Sea of Japan off central Honshu, seasonal change in the gonadosomatic index ( GSI ) was examined and histological observations of both ovaries and testes were conducted. The female GSI gradually increased from May, maintained a high level between August and October and then rapidly decreased. The histological observations of the ovary showed that vitellogenesis began from May and developed until September. Spent fish began to be observed in October and all the fish collected in December were spent. There was only one peak in the seasonal change in GSI of males and females in Wakasa Bay. Thus, yellow sea bream spawn only in autumn in Wakasa Bay. Differences in the reproductive cycle and number of spawning seasons were found between the two different latitudinal areas, Wakasa Bay and the East China Sea (low latitudinal area) where the spawning seasons were spring and autumn. An increase in water temperature is an important factor to initiate gonadal maturation (recrudescence) and is terminated by the short day-length in Wakasa Bay.     

Long-term stock assessment and growth changes of the Japanese sardine (Sardinops melanostictus) in the Sea of Japan and East China Sea from 1953 to 2006

We estimated the stock size of Japanese sardine ( Sardinops melanostictus ) in the Sea of Japan and East China Sea since 1953 using cohort analysis based on the changes of growth patterns. Growth of Japanese sardine, estimated by using annual rings on archived scales since 1961, showed that body lengths were extremely stunted in the 1980–1987 year-classes. The body length at age 3 from February to April in the 1980–1987 year-classes, a period when the stock size exceeded 4 million tons, was 180.0 ± 2.6 mm (mean ± SD), and in the other year-classes was 195.1 ± 7.6 mm. The body length at age 3 and wet weight of zooplankton in August in the offshore area of the Sea of Japan had a significantly positive correlation. We assumed three scenarios for maturation ratios, and estimated Ricker's spawner–recruitment relationships. We analyzed the correlations between logarithmic recruitment residuals (LNRR) and environmental factors in winter, represented by the North Pacific index (NPI), Aleutian low pressure index (ALPI), Pacific decadal oscillation (PDO), monsoon index (MOI), Arctic oscillation (AO) and Southern oscillation index (SOI). Significant correlations were observed between MOI and LNRR and between AO and LNRR. A combination of strong MOI and weak AO would increase the biomass of phytoplankton and zooplankton and subsequently increase the recruitment of Japanese sardine.     

Size,permeability and buoyancy of anchovy (Engraulis Encrasicolus) and sardine (Sardina Pilchardus) eggs in relation to their physical environment in the Bay of Biscay

The size and specific gravity of eggs of marine pelagic fish partly determine their dispersal and survival. Using an original dataset of anchovy and sardine eggs, sampled in spring over the last decade in the Bay of Biscay, we provide a parameterization of these properties on ambient water temperature and salinity. We used the density gradient column for measurement of egg specific gravity. The column was also filled with homogeneous water for sinking velocity experiments. For anchovy, these experiments confirm that the effect of egg permeability through the chorion could be neglected when modelling sinking, while it has to be considered for sardine, its perivitelline space representing 78.6% (±6.2%) of the total egg volume, as opposed to 5–10% for most teleosts species. We estimated a coefficient of permeability of the chorion of 0.0038 mm s^?1. However, permeability should not affect the measurement of sardine egg specific gravity in a gradient column, provided a minimum duration before reading is respected for equilibrium to be reached. In relation to their environment, we found that the egg specific gravity is largely determined by sea surface salinity for both species, whereas egg size is weakly but significantly impacted by temperature, for sardine only. On average, the estimated difference in specific gravity between egg and surface water is ?0.92 σ_T for anchovy and ?1.06 σ_T for sardine. The detailed parameterization of the relationship between eggs and water properties should prove useful, in particular to modellers dealing with the dispersal of fish early life stages.     

Influence of tropical seasonal changes on oocyte diameter, responses to hormonal induction and hatching quality in two strains of the catfish, Heterobranchus longifilis Val. (Clariidae)

A comparison was made between the oocyte diameter, pseudo‐absolute fecundity (PAF) and egg quality of two strains of the catfish, Heterobranchus longifilis, maintained in outdoor ponds under a tropical highland climate in Cameroon. The results indicated that circannual endogenous rhythms for ovarian recrudescence and regression are absent in H. longifilis, irrespective of breeding history, age and strain. There appeared to be a possible slight adverse effect of climate (dry season) on reproductive performance of both strains. Oocyte diameter and PAF increased with increasing fish weight when data from both strains were combined, indicating that the two strains may exhibit the same reproductive performance. Viable eggs were obtained all year round after hormonal induction. It is concluded that for propagation of H. longifilis in tropical African countries, broodfish could be maintained in outdoor earthen fish ponds with proper feeding.