Response Mechanism Of Winter-spring Cohort Of Neon Flying Squid To The Climatic And Environmental Variability In The Northwest Pacific Ocean

The neon flying squid(Ommastrephes bartramii) is widely distributed in the subtropical and temperate waters in the North Pacific Ocean and has importantly economic values. O. bartramii is a short-lived species, whose whole life cycle is closely linked to climatic and oceanographic conditions. The biological and physical environments are very complicated in the North Pacific, the interactions between the climatic and oceanic phenomena greatly affect the fish resources, especially the variability in the path and strength of the Kuroshio and Oyashio Current, the occurrences of the El Ni?o and La Ni?a events and the Pacific Decadal Oscillation(PDO), which strongly affect the fishing ground of O. bartramii. In recent two decades, annual catch of O. bartramii greatly fluctuated. One of the main reasons is the significant impacts of the variations of climatic and oceanic events on various spatio-temporal scales on the abundance and spatial distribution of O. bartramii in the North Pacific Ocean. However, the influence mechanism is still unknown. Therefore, we conduct a study to explore the response mechanism of winter-spring cohort of O. bartramii to the climatic and environmental variability in the Northwest Pacific Ocean, which can help the sustainable exploitation of squid stocks and provide bases for scientific fishery management.We assume that the large-scale climate variability significantly regulates the environmental variables on the spawning and fishing ground of O. bartramii in the North Pacific, the fluctuations in the regional environmental conditions directly drive the variability in incubation and feeding environments in the early life stage and habitat conditions during the adult stage, ulitimately leading the interannual spatio-temporal dynamic of squid stocks.Based on the hypothsis above, with the commercial fishery data from the Chinese distant-water squid-jigging vessels during 1995-2011 on the traditional fishing ground between 35o-50 oN and 150o-175 oE in the Northwest Pacific Ocean, as well as the environmental data and climatic index, we examine the spatio-temporal distribution of O. bartramii, identify and predict the squid habitat hotspots and analyze the interannual and seasonal variabilities in the squid abundance for exploring the response mechanism of abundance and fishing ground gravity centers of O. bartramii to the climatic variability. We further develop the models to predict the squid abundance and the distribution of fishing ground, and further explore the variability in the squid habitat suitability and the ocean primary productivity under different anomalous environments. Through the studies above-mentioned, we evaluate the impacts of climate variability and various environmental conditions on the fluctuation of squid fishing ground, which help understand the essence of the stock dynamics of O. bartramii. The results are summarized as follows:(1) Spatio-temporal distribution of O. bartramii and its habitat hotspots. The spatial distribution and abundance of O. bartramii exhibited interannual and seasonal variability. Annual latitudinal and longitudinal gravity centers were mainly located in the waters between 41.7° and 43.4°N and between 154.2° and 160.4°E, respectively. The nominal catch-per-unit-effort(CPUE) was from 1.36 t/d to 2.68 t/d during July to November, the highest CPUE occurred in August. The inferred optimal ranges for sea surface temperature(SST), sea surface height anomaly(SSHA), sea surface salinity(SSS), chlorophyll-a(Chl-a) concentration, mixed layer depth(MLD) and eddy kinetic energy(EKE), determined from the cumulative distribution curves, were 17.6-18.6oC,-5-1.5 cm, 33.58-33.79 psu, 0.41-0.55 mg/m3, 15.5-18.5 m and 28-35.5 cm2/s2, respectively. Potential habitat hotspots identified for O. bartramii coincided with locations of subarctic front zone(SAFZ). Approximately 72% of the average O. bartramii CPUE observations were found in the regions with probability indices higher than 0.6. Based on linear regression analysis, we found a statistically significant positive relationship between the Ln(CPUE+1) and probability indices over 1998-2007. The predicted areas of high CPUEs corresponded well with the fishery observation data in 2008 and 2009.(2) Interannual variability of the abundance of O. bartramii. The generalized linear model(GLM) and generalized additive model(GAM) were used to evaluate the interannual and seasonal variability in the abundance of O. bartramii, and to evaluate the influences of each variable on the CPUE. The results from GLM suggested that year, month, latitude, sea surface temperature(SST), mixed layer depth(MLD), and the interaction term(SST×MLD) were significant factors. According to the Akaike Information Criterion(AIC), the optimal model based on the GAM method included all the six significant variables and could explain 42.43% of the variance in nominal CPUE. The interannual variability in the abundance of O. bartramii was exhibited as follows: the variability in the CPUE from 1995 to 2002 was relatively low and exhibited slow decreased trend with small fluctuation, the CPUE was highest in 1996 and lowest in 2001 during this period. After 2002, the CPUE increased significantly and was much higher than that in the previous years. After a small decrease during 2004 to 2006, the CPUE in 2007 reached the highest value during the 17 years. The CPUE then subjected to a dramatic decrease, and the O. bartramii CPUE became the lowest in 2009. The CPUE in 2010 and 2011 were basically similar.(3) Impacts of the PDO on the O. bartramii abundance. Over the period of 1995-2011, the squid had experienced two full PDO cycles during which 5 El Ni?o and 8 La Ni?a events occurred. The warm PDO phases appeared over the periods of 1995-1998 and 2002-2006, and the cool PDO phases prevailed over the periods of 1999-2001 and 2007-2011. The squid catch and CPUE during the warm PDO phases were significantly higher than those during the cold PDO phases. The squid abundance was positively correlated with the PDO index(PDOI) at a one-year time lag. The same correlation was also evident between CPUE and Chl-a concentration anomaly on the spawning and fishing ground. Furthermore, an abnormally warm temperature on the fishing ground during the La Ni?a years over the warm PDO phase provided favorable oceanographic conditions for the habitat conditions for the growth of O. bartramii, whereas a lower temperature on the fishing ground during the El Ni?o years over the cold PDO phase generally corresponded to a low CPUE. The CPUE was closely related to the climate-induced variability of the large-scale circulation in the northwestern Pacific Ocean: high squid abundance often occurred in a year with a significant northward meander of the Kuroshio Current. The Kuroshio Current advected the warmer and food-rich waters into the fishing ground and multiple meso-scale eddies forming due to current instability enhanced the food retention on the fishing ground, all of which were favorable for the life stage development of the western squid stocks.(4) Impacts of the PDO on the spatial distribution of fishing ground of O. bartramii. Significant interannual and seasonal variability were observed in the longitudinal and latitudinal gravity centers(LONG and LATG) of fishing ground of O. bartramii over the period of 1995-2011. The annual average LATG mainly occurred in the waters between 40°-43.5°N. The LATG tended to occur in the southern regions on the fishing ground during the warm PDO phases and in the northern waters during the cold PDO phases, respectively. The annual average LATG mainly occurred in the waters between 153°-161°E. Correlation analyses suggested that the PDOI was significantly negatively related to the LATG of O. bartramii but was not significant correlated with the LONG. Moreover, both the SST anomaly(SSTA) and SSHA on the fishing ground were positively correlated to the LATG and negatively correlated to the PDOI, but no significant relationships were found between the MLD anomaly(MLDA) and squid distribution(LONG and LATG), as well as the PDOI. The apparent north-south spatial shift in the annual LATG appeared to be associated with the PDO phenomenon and was closely related to the SST and SSH on the fishing ground, whereas the MLD might contribute limited impacts to the distribution pattern of O. bartramii. The warm PDO regimes tended to yield cold SST and low SSH, resulting in a southward shift of LATG, while the cold PDO phases provided warm SST and elevated SSH, resulting in a northward shift of LATG.(5) Response mechanism of the PDO affecting the O. bartramii abundance. Taking the period of 2002-2011 for example, we examined variations in the environmental conditions on the spawning ground for recruitment, preferred ranges of environmental variables, fishing effort distributions and habitat hotspots on the fishing ground during different PDO phases. The PDO was found to play a crucial role in regulating the biophysical environmental conditions related to O. bartramii. The interaction between SST and suitable spawning zone(SSZ) considered as an indicator of incubation condition was not sufficient to explain the recruitment variability. However, the changing Chl-a concentration caused variation in feeding environment for squid paralarvae and juveniles, primarily influencing the O. bartramii recruitment. Comparing to the cold PDO phase, high frequency of fishing effort occupied the regions with lower SST and relatively enhanced Chl-a concentration during the warm PDO phase. Fishing efforts tended to be intensive and shifted westward and northward in the cold PDO phase. Moreover, the warm PDO yielded prominently enlarged squid habitat hotspots. This study suggested that stock level of western winter-spring cohort of O. bartramii can be explained by food availability on the spawning ground, SST and Chl-a concentration on the fishing ground and fishing effort distributions through the large-scale PDO climate variability interacted with local environmental conditions in the Northwest Pacific Ocean.(6) Variability of the habitat suitability of O. bartramii under different environments. An integrated habitat suitability index(HSI) model was developed to evaluate the variability of suitable habitat for O. bartramii under anomalous environments during July to November in 1998, 2008 and 2009 based on two empirical methods(the arithmetic mean model, AMM; the geometric mean model, GMM). The AMM model was found to perform better than the GMM model. The AMM-based HSI model was further used to predict the HSI values on the squid fishing ground. The predicted HSI values in 1998(high catch), 2008(average catch) and 2009(low catch) indicated that the squid habitat quality was strongly associated with the ENSO-induced variability in the oceanic conditions on the fishing ground. The La Ni?a events in 1998 tended to yield warm SST and favorable range of Chl-a concentration and SSHA, resulting in high-quality habitats for O. bartramii. While the fishing ground in the El Ni?o year of 2009 experienced anomalous cool waters and unfavorable range of Chl-a concentration and SSHA, leading to relatively low-quality squid habitats. Our findings suggest that the La Ni?a event in 1998 tended to result in more favorable habitats for O. bartramii in the Northwest Pacific with the gravity centers of fishing efforts falling within the defined suitable habitat and yielding high squid catch; whereas the El Ni?o event in 2009 yielded less favorable habitat areas with the fishing effort distribution mismatching the suitable habitat and a dramatic decline of the catch of O. bartramii.(7) Variability of the net primary productivity on the fishing and spawning grounds of O. bartramii under different environments. The net primary productivity on the fishing ground exhibited obviously seasonal variability in the longitudinal direction, which was low in spring and winter and high in summer and autumn. In the fishing months from July to November, the suitable range of the net primary productivity for O. bartramii was 500-700 mg C/m2/d in July, 500-800 mg C/m2/d in August, 500-1000 mg C/m2/d in September, 500-800 mg C/m2/d in October and 300-500 mg C/m2/d in November. The most suitable net primary productivity in July, August, September, October and November was 700 mg C/m2/d, 600 mg C/m2/d, 700 mg C/m2/d, 600 mg C/m2/d and 400 mg C/m2/d, respectively. Significant positive relationship was found between the average latitude of the most suitable net primary productivity and the latitudinal gravity center of fishing effort from July to November, suggesting that the locations of fishing effort were not randomly distributed, which might be strongly affected by the latitudinal distribution of the most suitable net primary productivity. The annually squid abundance was positively related to the net primary productivity in March as well as its average value in July to November. It was inferred that annual squid stock level was driven by the interaction between the concentration of net primary productivity on the spawning ground in March and on the fishing ground from July to November. Our findings indicated that the anomalous environments(El Ni?o and La Ni?a events) had strong influences on the variability of the net primary productivity on the spawning and fishing grounds, however, its regulating mechanism tended to vary.Overall, the abundance and spatial distribution of fishing ground of O. bartramii exhibited significant seasonal and interannual variability, which were strongly related to the climate regime shift in the North Pacific Ocean. When the climate regime made a shift, the environments on the spawning and fishing grounds would be changed, which led to the vaiability in the recruitment and habitat hotspots, and further resulted in the fluctuations in the squid abundance and distribution.