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

State of Saldanha Bay & Langebaan Lagoon 2011
Fish
in Saldanha and Langebaan. Results from angler surveys indicate that approximately 92 tons of
white stumpnose is landed by anglers each year (Næsje et al. 2008). Further details of the results of
these studies were reported on in the State of the Bay 2008 report (AEC 2009). The research on sand
sharks suggests that the common sand shark species in bay and lagoon is actually Rhinobatos blockii,
not R. annulatus as previously thought (Dunn & Schultz UCT Zoology Department personal
communication). New information on the life history of this species has been collected and will be
published in the near future.
The Saldanha Bay Water Quality Forum Trust (SBWQFT) commissioned AEC to undertake
experimental seine-net sampling of near shore fish assemblages at a number of sites throughout the
Saldanha-Langebaan system during 2005, 2008, 2009, 2010 and 2011 as part of the monitoring of
ecosystem health “State of the Bay” programme. In the 2006 report it was noted that the existing
seine-net survey data was the most suitable for comparative analyses over time and it was
recommended that future seine-net surveys were conducted during late summer - early autumn, as
this was the timing of peak recruitment of juveniles to the near-shore environment, as well as the
timing of most of the earlier surveys. Since 2008, seine-net surveys have therefore been conducted
during March-April of each year. These studies have made a valuable contribution to the
understanding of the fish and fisheries of the region.
This report presents and summarizes the data for the 2011 seine-net survey and investigates
trends in the fish communities by comparing this with data from previous seine-net surveys
(1986/87, 1994, 2005, 2007, 2008, 2009 & 2010) in the Saldanha- Langebaan system. Recent data on
the commercial and recreational catch-per-unit-effort of white stumpnose (the principal target
species in the Bay) are also presented and compared to the results of the experimental seine net
surveys.
9.2 Methods
9.2.1.1 Field sampling
Experimental seine netting for all surveys covered in this report was conducted using a beach-seine
net, 30 m long, 2 m deep, with a stretched mesh size of 12 mm. Replicate hauls (3-5) were
conducted approximately 50 m apart at each site during daylight hours. The net was usually
deployed from a small rowing dinghy 30-50 m from the shore. Areas swept by the net were
calculated as the distance offshore multiplied by the mean width of the haul. Sampling during 1986-
87 was only conducted within the lagoon where 30 hauls were made, whilst 39 and 33 replicate hauls
were made at 8 and 11 different sites during 1994 and 2005 surveys respectively in the Bays and
Lagoon. During 2007, 21 hauls were made at seven sites in the both Bays and Lagoon and for the last
four years, 2-3 hauls have been made at each of 15 standard sites every April (2008-2011) (Figure
9.1). Large hauls were sub-sampled at the site, the size of the sub-sample estimated visually and the
remainder of the catch released alive.
9.2.1.2 Data analysis
Numbers and mass of fish caught were corrected for any sub-sampling prior to data analyses. All fish
captured were identified to species level where possible and abundance calculated as the number of
fish per square meter sampled. During the six most recent seine-net surveys (2005, 2007, 2008,
2009, 2010 & 2011) the total of each species caught was weighed to the nearest gram. The weight of
any fish released alive was calculated from published length-weight relationships (Mann 2000). For
the purposes of this report, abundance data were used for analysis of spatial and temporal patterns.
The number of species caught, average abundance and associated variance of fish (all species
combined) during each survey were calculated and graphed. The average abundance of the most
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