appendix b final 2008 biological surveys of los angeles and long ...

4.0 Ichthyoplankton
The ichthyoplankton stations were designated as being in one of three general areas (Table
4.4-2): deep outer harbor (seven stations), shallow outer harbor (four stations), and inner harbor
(eight stations). The average weighted mean abundance of larval fish in each of these three
areas was: shallow outer harbor 1,523/100m 2 , inner harbor 1,297/m 2 , and deep outer harbor
1,157/100m 2 . The range of values in each area was variable; for example, the value for the
shallow outer area was the highest based on the large number of larvae found at station LA7
(4,381/100m 2 in Tables 4.3-3 and 4.3-5). The other three stations designated shallow outer
were much lower ranging from 257 to 588 larvae/100m 2 .
4.5 METHOD COMPARISON STUDY RESULTS
The comparison of the two sampling approaches was based on two analyses. First, total larval
abundance (#/100m 2 ) in paired samples (three-net sampling vs. CalCOFI-type sampling - Table
4.5-1) was compared by calculating the difference between the two samples and testing the
hypothesis that the average difference was zero. Since the differences were not normally
distributed, abundance data were transformed (log 10 ) prior to calculating differences. As Table
4.5-1 shows, the three-net method overall yielded somewhat higher abundances than the
CalCOFI method, although in five of the twenty sample pairs abundances were lower in the
three-net samples. The statistical analysis showed that the average difference was not
significantly different from zero, but a power analysis showed that due to the small sample size
and variation in the data, there was only a 7 percent chance that a difference from zero could
have been detected. Since acceptable levels of power for such tests are 70 percent or greater,
the results of this analysis do not support a comparison of the two methods.
The second analysis compared the Bray-Curtis similarity between the paired samples collected
using the two methods. The average similarity between the paired samples at the ten stations
was 56.8 percent (Table 4.5-1). The average Bray-Curtis similarity among the 171 possible
sample pairs based on the average abundances for the three surveys at all 19 stations was
slightly higher, at 57.3 percent. The average similarity between paired samples at the ten
stations would be expected to be much greater than the average based on a random sample of
all of the samples if the CalCOFI-type sampling was effective at replicating the sampling done
with the three nets. Although the result of the statistical comparison was unable to detect a
statistically significant difference between the two methods, the overall results were
inconclusive.
Due to the patchiness of plankton, filtering larger volumes of water increases the chance of
collecting a more representative cross section of the ichthyoplankton composition, but will also
increase the sample processing time and cost. It is recommended that for future studies in the
Port complex the current three-net method be continued with a modification to the midwater
collection. It is suggested that the surface and epibenthic sampling continue without change.
These techniques allow more water in these two strata to be filtered than does the CalCOFI
technique. This increases the chance of collecting rare taxa that might be in higher
concentrations in these two strata than in the midwater. However, it is recommended that the
‘steps’ in the current midwater sampling be eliminated. These ‘steps’ introduce potential biases
into the data by oversampling the depth strata where the net was during its ‘steps’. It is also
impossible to replicate the exact depth strata used at each station or between surveys at a
particular station due to factors such as vessel, wind, current speeds, and length of tow cable.
Therefore, it is recommended that during any future midwater collection the net be deployed in a
similar fashion to that used during the CalCOFI-type tows in this study, where the nets
deployment and retrieval speeds through the water column be constant so that all depths are
evenly sampled.
2008 Biological Surveys of Los Angeles and Long Beach Harbors 4–7
April 2010