The relative abundance of adult Argentine anchovies (Engraulis anchoita) and the thermal structure of the sea between 41° and 45°30′S during four research surveys were compared. Acoustic data were collected while making parallel transects along which CTD (conductivity, temperature, depth) stations were regularly distributed. Anchovy abundance was related to both the sea surface temperature and the stratification of the water column, as classified according to the φ parameter of stability (Simpson, 1981). Regarding the whole water column, the sea temperatures where adult anchovies were recorded ranged from 8.5 to 16.5°C, but anchovy echo traces from waters above 14°C were obtained mainly at night. The sea surface temperature bounds for anchovy distribution were 11 and 17°C, with a preferential range between 12.5 and 16°C, but no absolute value of sea surface temperature was found to be most favourable for the species. The highest fish abundances were related to sharp thermal gradients, either horizontally recorded in frontal zones (≈ 0.02°C or more per km) or vertically associated to the occurrence of a thermocline (stability > 10 J m–3). Sea fronts seemed to be the axes for the distribution of anchovy shoals, and the annual changes in the positions of the fronts seemed to be followed by analogous changes in the locations of the main anchovy concentrations. Within thermally stratified areas and during the daytime, anchovies apparently preferred the thermocline layer or the layer immediately above that, although a few fish shoals were located below the thermocline.
The mid‐shelf front (MSF) of the Buenos Aires province continental shelf in the Southwestern Atlantic Ocean plays a central role in the pelagic ecosystem of the region acting as the main spring reproductive area for the northern population of the Argentine anchovy Engraulis anchoita and supporting high concentrations of chlorophyll as well as zooplankton, the main food of anchovy. To investigate the influence of environmental variability on the reproductive success of E. anchoita, we analyzed a 13‐yr time series (1997–2009) of environmental data at MSF including chlorophyll dynamics, as well as zooplankton composition and abundance, ichthyoplankton distributions, and recruitment of E. anchoita. Spring chlorophyll concentrations showed high interannual variability and were mainly influenced by changes in water temperature and vertical stratification, which in turn control nutrient supply to the surface. Chlorophyll dynamics (magnitude, timing, and duration of the spring bloom) explained most of the variability observed in E. anchoita recruitment, most likely via fluctuations in the availability of adequate food for the larvae. Our results suggest that satellite ocean color products can be valuable tools for understanding variability in ecosystem dynamics and its effects on the recruitment of fish.
The Daily Egg Production Method (DEPM) was used to compute spawning biomass of Engraulis anchoita off Argentina. Estimates of the daily egg production (P 0 ) for the northern stock ranged from 594 to 936 eggs m 22 , whereas the annual means of the Patagonian stock ranged from 185 to 605 eggs m 22 . The mean values estimated for the Argentine anchovy DEPM parameters were characterized by inter-annual differences greater than inter-regional differences with the exception of P 0 . For the northern population, the estimates of mean weight of mature females ranged from 15 to 26 g, the relative batch fecundity from 414 to 600 eggs g 21 , the spawning frequency (S) from 0.078 to 0.179, and the females ratio from 0.519 to 0.622 of the spawning stock. The estimates corresponding to the Patagonian stock were similar being 15 to 24 g, 418 to 583 eggs g 21 , 0.079 to 0.244, and 0.394 to 0.590, respectively. The annual estimates of the spawning biomass for the northern and Patagonian populations ranged between 1.6 -3.5 and 0.3 -1.5 million metric tons, respectively. A weakness in the application of DEPM was the low precision in the estimation of the daily egg production and the spawning fraction. Alternative methodologies to increase the precision of P 0 and S are discussed. In spite of the high variance of the spawning biomass estimates, the use of DEPM offers valuable information to adjust acoustic estimates and provides time series of anchovy population size and biological parameters for basic research.
Aerial line transect surveys of the density of humpback whales (Megaptera novaeangliae) conducted off West Greenland eight times between 1984 and 2007 were used to estimate the rate of increase on the summer feeding ground. Only surveys in 1993, 2005 and 2007 had enough sightings to construct independent density estimates, whereas the surveys in 1984–85 and 1987–89 had to be merged and treated as two surveys. The annual rate of increase was 9.4% yr–1 (SE = 0.01) between 1984 and 2007. This rate of increase is higher than the increase estimated at the breeding grounds in the West Indies, but is of the same magnitude as the observed rate of increase at other feeding grounds in the North Atlantic. A matrix model based on observed life history parameters revealed that the theoretical growth rate of a humpback whale population ranged between 1 and 11%. This confirms that the observed growth in West Greenland is within the plausible values. The survey in 2007 was used to make a fully corrected abundance estimate including corrections for whales that were submerged during the passage of the survey plane. The line transect estimate for 2007 was 1,020 (CV = 0.35). When the estimate was corrected for perception bias with mark-recapture distance sampling (MRDS) methods, the abundance increased to 1,505 (0.49). A correction for availability bias was developed based on time-depth-recorder information on the time spent at the surface (0–4m). However, used directly this correction leads to a positively-biased abundance estimate and instead a correction was developed for the non-instantaneous visual sighting process in an aircraft. The resulting estimate for 2007 was 3,272 (CV = 0.50) for the MRDS analysis. An alternative strip census estimate deploying a strip width of 300m resulted in 995 (0.33) whales. Correction for perception bias resulted in 991 (0.35) whales and corrected for the same availability bias as for the MRDS method resulted in a fully corrected estimate of 2,154 (0.36) humpback whales in West Greenland in 2007.
Relative batch fecundity, spawning frequency, size at 50% maturity, sex ratio, and egg production were estimated for the Bonaerense and Patagonian Engraulis anchoita stocks. Samples were collected during 19 research surveys carried out over the main spawning seasons and areas of both groups from 1993 to 2008. Reproductive potential between stocks was compared by analyzing the length-class structure and considering environmental variations. Daily egg production varied between 0.2 × 10 7 and 7.3 × 10 7 oocytes and the mean annual value was higher for the Bonaerense than the Patagonian stock (2.9 × 10 7 and 0.54 × 10 7 oocytes, respectively; P < 0.019). In both stocks, larger size classes produced more oocytes than smaller and more abundant females. No correlation was observed between egg production and adult biomass and between egg production and age-1 recruits. Length at first maturity was lower for the Bonaerense than the Patagonian stock (100.3 and 119.8 mm total length, respectively; P < 0.001). Bonaerense and Patagonian spawning frequency estimates were 0.125 and 0.116 (statistically significant differences were not observed, P > 0.788), and mean relative batch fecundity values were 508 and 493 oocytes g-1 (P > 0.680), respectively. Mean annual relative batch fecundity values for the Bonaerense stock showed a negative trend (P < 0.016) in the period 1993-2008, coinciding with a 4 ºC increase in mean sea surface temperature in the area during the same period.
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