State of the Bay Report 2011-Final.pdf - Anchor Environmental

State of Saldanha Bay & Langebaan Lagoon 2011
Fish
A two way PERMANOVA indicated significant differences in the fish community between
sample years (Pseudo F = 2.7, P< 0.0001) and between sites (Pseudo F = 9.2, P< 0.0001) and a
significant interaction effect (Pseudo F = 2.9, P< 0.0001). Pairwise tests indicate that there are
significant differences between all years sampled and at least two of the other annual sampling
events (Table 9.4). This is indicative of the high natural variability in the surfzone fish community
inhabiting Small Bay. We suspect, given the orientation of Small Bay facing into the prevailing
southerly wind, that short term meteorological changes, (such as a wind change to a northerly or
westerly) that are common during April and strongly influence the nature of the surfzone, are the
primary drivers of this observed variability. The only way to account for this natural variability is to
sample more intensively, by replicating the survey within years, sampling the same sites on different
days within each survey and repeating it over several weeks. This is unfortunately not logistically
possible, but trends in the ecological health of the Bay (for fish at least) can still be inferred from any
long term consistent trends over time. It is clear from the MDS plot that only some of the 2005 and
2011 samples collected in Small Bay were the most dissimilar from the other annual samples and
there is no consistent trend that may be indicative of increasing or decreasing ecosystem health
(Figure 9.8). It must be noted that as with the 2005 samples, there is also high inter-sample
variability (spread of data in the MDS plot) within the 2011 samples. (Some samples group with
those from other survey periods, some are outliers). The 2011 sites that are outliers include the
small craft harbour (typically different from the other Small Bay sites) and the Campsite sites (where
very high catches of harders were made during 2011 sampling), and this variability does not
necessarily represent declining ecosystem health at these sites.
Although the 2011 Small Bay samples were not significantly different from the 2010
samples, the fish community overall was significantly different from that sampled during 1994, 2007
and 2009. SIMPER analyses identified higher abundance of harders, and decreased average
abundance of silversides, white stumpnose, Cape sole, blacktail and gobies in the 2011 samples as
the dominant causes (>80%) of dissimilarity between the 2011 samples and the significantly
different 1994, 2007 and 2009 samples. Although none of these species had disappeared from
catches in Small Bay during 2011, they were on average substantially less abundant than in nearly all
of the earlier surveys and this is somewhat concerning.
Table 9.4.
Results of the multivariate PERMANOVA pairwise tests between Small Bay fish samples
collected in different years. NS: not significant, *: P < 0/05, **: P < 0.01
1994 2005 2007 2008 2009 2010
1994
2005 *
2007 ** NS
2008 NS * NS
2009 ** NS NS *
2010 NS * NS * *
2011 * NS * NS * NS
Within Big Bay too, little grouping of sampling years in the MDS plot is evident with the 2008
and 2005 outliers representing a few of Plankiesbaai and North Bay samples (Figure 9.9). All of the
2011 samples are distributed well within the range of samples collected in earlier years, indicating
no substantial changes in the Big Bay fish communities overall at sampled sites. The mixed model
PERMANOVA test did however, indicate significant differences between sites (Pseudo F = 7.6, P<
0.001), between sampling events (Pseudo F = 14.9, P< 0.05) and a significant interaction effect
(Pseudo F = 4.1, P< 0.001). Pairwise testing showed that only the Big Bay fish samples collected
during 1994 were significantly different from those collected during 2007 and 2009. Big Bay fish
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