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

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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 194 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Fish common fish species caught in the three main areas of the system, namely Small Bay, Big Bay and Langebaan lagoon during each survey, were similarly calculated and presented graphically. The average abundance of the four-five most ubiquitous species in the system over all survey years was calculated and plotted for each sampling site. In order to investigate changes in the entire fish community composition between years, multivariate statistical analysis were conducted using the PRIMER software. Fish density data were fourth-root transformed and the Bay-Curtis similarity index was used to create similarity matrices. Relationships between years were represented using multidimensional scaling and these were statistically tested using two way mixed model PERMANOVA tests with years as a fixed effect and sites as a random effect (this takes into account the variability between sampled sites when comparing samples between years). The principal species contributing to dissimilarities between years were identified using the SIMPER routine. The status of the most significant commercial and recreational fishery in the Saldanha Bay system (that for white stumpnose), i.e. the health of the adult stock as opposed to the juvenile recruitment that is assessed by the experimental seine net fishery, was investigating using data from three different sources: 1. Commercial catch returns from traditional line fish permit holders active throughout the Saldanha Bay – Langebaan area over the period January 2006 – December 2011. These data were obtained from the Department of Agriculture, Forestry and Fisheries (DAFF). 2. Roving creel surveys of shore anglers and slipway boat inspections conducted by the SANParks Coastcare programme with the assistance of Prof CG Attwood (UCT) over the period January 2006- January 2009. These data were obtained from Prof. Attwood. 3. Boat landing site inspections by scientific fishery observers acting on behalf of the DAFF over the period September 2007 to September 2010. These data were obtained from the DAFF. These data include information on the number of fishers (crew or shore anglers in the group), the hours fished and the catch of each species. The commercial permit holder is legally required to complete the daily catch return, but these are not frequently validated (i.e. are reliant on the honesty of the permit holder). Slipway inspections by trained monitors are a method of validating commercial catch returns, but include data from both the commercial and recreational boat fishing sectors. All these data are fisheries dependent – i.e. not an independent scientific survey (such as the seine net survey) and therefore reflect the behaviour of the fishery (targeting, gear and catch restrictions etc) as well as the relative abundance (or availability to the fishery) of the adult stock. Assessing trends in white stumpnose catch rates from all three different sectors , commercial boat, recreational boat and shore anglers using data collected or reported by different groups does increase the robustness of the analysis and confidence in the results (should they be in general agreement!) Catch-per-unit-effort (CPUE) was used as an estimate of relative abundance of adult (above the minimum size limit) white stumpnose in the Saldanha-Langebaan system. The rational is that the more abundant the species is in the fished area, the more will be caught per unit fishing time. This was simply calculated as the number or weight of white stumpnose caught per angler-hour fished (total catch or weight divided by the number of anglers/crew multiplied by the hours fished). The average monthly CPUE for each data set was plotted to investigate any trends in the relative abundance over time. The average annual catch rate, calculated from July to June in order to encompass the summer fishing season, was also graphically compared to the average annual catch of juvenile white stumpnose in the seine net surveys. 195 ANCHOR e n v i r o n m en t a l
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