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

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State of Saldanha Bay & Langebaan Lagoon 2011 Fish 9.3.2 Description of inter-annual trends in fish abundance and current status of fish communities in Small Bay, Big Bay and Langebaan lagoon Within the Saldanha-Langebaan system, harders, silversides and gobies numerically dominated the catches for all surveys. Overall the catches made during the 2011 survey were the largest on record within Small Bay (exclusively due to the very big catches of harders made at the three sites along the northern shore) and well above average for the Big Bay (second highest on record after the 2007 survey) and Langebaan lagoon sites (3 rd highest on record) (Figure 9.3). Estimated white stumpnose, nude goby and blacktail abundance, that was above average in Small Bay during the 2007 and 2008 surveys has remained below historical levels in this region since 2009 (Figure 9.4). Within Big Bay and Langebaan Lagoon however, higher than average fish density was again observed during the 2011 sampling. During the 2009 survey, the densities of all the more common fish species in Small and Big Bay were lower than the preceding two years and in some cases the lowest recorded during sampling thus far. The 2011 survey saw a recovery in the density of harders, white stumpnose, elf and silverside in Big Bay, whilst within Langebaan lagoon, abundance of harders, Caffrogobius sp. and white stumpnose was well above the average recorded in earlier surveys (Figure 9.4). With the exception of harders, the opposite trend was observed in Small Bay i.e. a decrease in abundance of the common species (Figure 9.4). It appears that the unfavourable environmental conditions that reduced the spawning success of adults and caused high mortality rates of eggs, larval and juveniles of several species during the 2008-2009 periods have passed and the results of better than average recruitment are seen in the 2010 and 2011 data for Big bay and Langebaan Lagoon. The observed density of white stumpnose and harders in Langebaan lagoon recorded during the 2010 and 2011 sampling remains higher than that recorded during most of the earlier surveys, suggesting good recruitment in this area and possibly reflecting the demonstrated benefits of the Langebaan Lagoon marine protected area for exploited fish species (Figure 9.4). Naturally high variability in recruitment strength is frequently observed for marine fish species and it is likely that natural environmental fluctuations rather than anthropogenic factors that caused the poor recruitment in 2009. The fact that Small Bay showed the opposite trend in recruitment strength compared to the Big Bay and Lagoon sites during 2011, is however, cause for concern. The better than average recruitment recorded at the latter sites suggests that it was not a “poor” year for egg, larval and juvenile survival within the Bay as a whole. Either the environmental conditions were not conducive for the dispersal of eggs and larvae into the Small Bay area (i.e. they didn’t get there), or it was not good for their survival (i.e. they got there but survival was poor). Both are plausible explanations, but if the environment was not conducive to survival of juveniles during the summer of 2010/2011 this could have been a result of anthropogenic factors. 202 ANCHOR e n v i r o n m en t a l
Fish abundance (No.m -2 ) State of Saldanha Bay & Langebaan Lagoon 2011 Fish 70 1986 1994 2005 2007 2008 2009 2010 2011 60 50 40 30 20 10 0 Figure 9.3. Small Bay Big Bay Lagoon Average fish abundance (all species combined) during eight seine-net surveys conducted in Saldanha Bay and Langebaan lagoon. (Error bars show one Standard Error of the mean). 203 ANCHOR e n v i r o n m en t a l
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