170 



Fishery Bulletin 88(1). 1990 



46°S 



62°W 



Figure 4 



Argentine hake CPUEs (in hundreds of kg/hour) in the southern 

 spawning area, south of Peninsula de Valdes, Argentina, Decem- 

 ber 1978. Isolines correspond to surface-to-bottom temperature 

 difference (°C), an indicator of water-column stratification; small 

 values indicate well-mixed conditions. 



I therefore examined data from egg and larval surveys 

 carried out in early spring and summer of li)78 (Cie- 

 chomski et al. 1979b). Ciechomski and Sanchez (1983) 

 established a well-defined relation between egg den- 

 sities and corresponding adult concentrations. 



In early spring (October through mid-November), 

 there are two distinct centers of hake spawning: the 

 northern catch between 36° S and 39° S and the south- 

 ern between 42°S and 44°S, in waters < 100 m deep 

 (Ciechomski et al. 1979b). It is not clear what propor- 

 tion of the fish spawn in each center, and whether fish 

 that spawned in the northern center continue migrat- 

 ing south or, instead, move directly offshore. 



Later in the season (December-January), several 

 surveys showed that the area 42-47° S becomes the 

 main spawning region. The largest concentrations of 

 hake eggs and larvae in December 1978 were found in 

 shallow waters at 44°S (Ciechomski et al. 1979b). 

 Figure 4 displays the hake CPUEs obtained in the 

 spawning area south of Peninsula de Valdes, during 

 cruise 9 (December 1978) of the Shinkai Maru. 



In this area, features known as tidal fronts have been 

 described by Carreto et al. (1986) and Glorioso (1987). 

 The fronts separate stably-stratified waters offshore 

 from well-mixed waters inshore. Vertical stratification 

 is generated by surface heating; the well-mixed zone, 

 on the other hand, results from stirring by strong tidal 

 currents and winds (Carreto et al. 1986). 



Water column stratification can be described by 

 surface-to-bottom temperature differences, plotted on 



Figure 4. However, the transition from a well-mixed 

 to a stratified regime and, thus, the location of the tidal 

 front, are not apparent in this figure: temperature dif- 

 ferences are >4°C, except in very shallow regions. This 

 may have been due to the development of a shallow 

 thermocline (~10 m) in inshore waters following a few 

 days of calm weather. The location of the front is in- 

 dicated instead by the transition from a shallow to a 

 fleeper (~35 m) thermocline (data not shown) which 

 occurred in the vicinity of water column temperature 

 differences of 7-8°C, as depicted in Figure 4. Highest 

 hake CPUEs were observed near this transition; in con- 

 trast, hake CPUEs inshore of the front were very low. 

 The last portion of the migration, which closes the 

 circuit, is the northward movement. This stage begins 

 in February and lasts through May-June, when hake 

 reach the wintering grounds. Fishing-effort maps sug- 

 gest that the fish move north and across the shelf up 

 to 40° S during February, and then continue north- 

 wards along the shelf break. 



Spatial statistics 



The spatial statistics are intended to represent "centers 

 of mass" of the distributions of fishing effort or hake 

 relative abundance during each month. Due to the un- 

 certainty in effort location reports in 1979, the centers 

 of fishing effort and hake CPUE are shown only for 

 1978 (Fig. 5). 



The average positions of CPUE and effort showed 

 cyclical north-south and east-west movements during 

 a year. The monthly centers of CPUE and effort were 

 quite similar, implying coincidence in the spatial dis- 

 tributions of fishing effort and apparent hake abun- 

 dance. This suggests that the fleet was successful in 

 locating dense aggregations of hake and lends con- 

 fidence to the use of fishing-effort data to follow the 

 commercial densities of migrating hake. 



The multiple centers of effort in December, January, 

 and February may be related to the existence of sepa- 

 rate spawning groups of hake. On the other hand, the 

 two centroids may merely reflect operational tactics 

 of the fleet; although effort has been standardized to 

 take into account differences in catching power, smaller 

 vessels may lack the endurance or hold capacity re- 

 quired for a trip to the southern spawning grounds. 

 Alternatively, fishermen may perceive that the catch 

 rates in the northern and southern grounds are not so 

 different as to make the longer trip worthwhile. 



Speed of hake migration 



Distances between monthly centers of effort and esti- 

 mated speeds of migration are shown in Table 1 . There 

 was a bimodality in estimated displacement speeds 

 throughout the year; the two modes corresponded. 



