196 



Fishery Bulletin 89(2), 1991 



Figure 1 



Location of sampling blocks. Open coast blocks are (1) San 

 Onofre, (2) adjacent to Agua Hedionda Lagoon, (3) Torrey 

 Pines, and (4) adjacent to Mission Bay. The two bays sampled 

 are Agua Hedionda Lagoon and Mission Bay, with sampling 

 blocks denoted. 



For further description of the sampling design and the 

 habitats see Kramer (1990). 



Three gear types were used, all lined or made of 

 3-mm mesh: a 1.0-m wide beam trawl, a 1.6-m wide 

 beam trawl, and a 1 x 6-m beach seine. The 1.6-m beam 

 trawl, set from a 15-m research vessel, was used to 

 sample the open coast and Mission Bay (Fig. 1). I 

 sampled Agua Hedionda Lagoon and the areas of Mis- 

 sion Bay that were inaccessible to the larger vessel with 

 the 1.0-m beam trawl, set from a 6-m skiff. The 1.0-m 

 beam trawl and the beach seine were pulled along the 

 bottom by two people to sample the shallow shoreline 

 (< 1 m) in the bays. The trawls were fitted with a wheel 

 and revolution counter to determine the distance 

 traveled by the trawl along the bottom, allowing a 

 quantitative assessment of fish density since the trawls 

 had a fixed mouth opening (Krygier and Horton 1975). 

 All trawls and seines were fished during the day. 



Table 1 



Gear weighting coefficients and their variances by length-class 

 for conversion of shoreline collections by beach seine and 1-m 

 beam trawl. Coefficients determined by 3-way ANOVA be- 

 tween gear types, blocks, and months of sample on density 

 for each length-class. Correction terms are given for length- 

 classes with significant gear effects (P< 0.05). There were no 

 significant gear effects in the 1.0-1. 6m beam trawls for open 

 water tows. 



Length class 

 (SL, mm) 



Correction term 



Variance 



26-30 

 31-35 

 36-40 



41-45 



3.291 

 4.398 

 2.752 

 4.699 



0.124 

 0.319 

 0.099 

 0.359 



All flatfishes taken in trawls and seines were mea- 

 sured to standard length (SL) in mm. Density of halibut 

 in 5-mm standard length-classes was determined for 

 juveniles <70mm SL. The thirteen length-classes used 

 were: SL<10mm, ll-15mm, 16-20mm, continuing to 

 66-70 mm SL. Abundance was determined by multiply- 

 ing the mean density for each habitat by the area of 

 each habitat. 



Gear comparison 



Densities based on the 1.0-m beam trawl collections did 

 not differ significantly from those of the 1.6-m beam 

 trawl for any length class (ANOVA, P>0.05, n 826). 

 However, the beach seine captured significantly fewer 

 small halibut (26-45 mm SL) than the 1.0-m beam trawl 

 (Table 1). Since significant biases existed, density and 

 abundance estimates of halibut were corrected for the 

 differences in gear efficiency by weighting the mean 

 density and variance for each length class where signifi- 

 cant differences in catchability were found (Table 1). 

 The weighted mean density for each gear type was 

 calculated as 



d w = (d 1 + gd,)/(l + g) 



where dj = unweighted density, d 2 = weighted density, 

 and g = weighting coefficient. 



Estimated variance of the weighted mean d w was 

 calculated as 



V(d w ) = V(d 1 ) + g 2 V(d 2 ) + d 2 2 V(g) + V(g)V(d 2 ) 



where V(dj) = variance of unweighted density, V(d 2 ) 

 = variance of weighted density, and V(g) = variance of 



