Northern Suburbs Branch, 15 May2024
We could not use our usual venue for the May talk and moved to Mildenhall in Duncraig. It took a little time to get set up, but with the help of our Tech expert, Graham Ezzy, a portable screen, and supper afterwards, all was well.
Our guest speaker was Mr. Peter Stephenson, a retired Principal Research Scientist, Stock Assessment, from the Department of Fisheries, Western Australia. He mentioned some of his work with Fisheries, including video footage of dolphins swimming into trawling nets, feeding and leaving again.
However, the main content was determining finfish stocks using the Pink Snapper at Shark Bay, the Red Emperor from the Pilbara and the Scalloped Hammerhead Shark in the Gulf of Mexico. Peter used a number of mathematical graphs and equations to explain changes in fish stocks.
Looking at these data from reported fishing vessel catch rates (adjusted for efficiency increases in brackets), it may seem dire for the fish stock. However, there are a number of other variables to take into account. Apart from the reliability of catch rates reported, as numbers fall, the fish have faster growth and increased productive rates. A common standard is that a 60% reduction in the stock size is OK.
Data for the Red Emperor in the Pilbara were collected from trawling vessels beginning in 1983. Red Emperors live for up to 40 years, determined by counting the bands on otoliths (ear bones), in a similar way to growth rings in trees. The increased market price of this species has led to stock size being below the target level from 2008 to 2015.
The Reproductive cycle of Pink Snapper in the Shark Bay area, which is listed as a World Heritage Site, is interesting in that the oceanic stock will travel to Cockburn Sound for breeding, whereas both the Denham Sound and Freycinet Estuary stock remain in these areas for life.
Surveys of the Denham Sound fish stock levels began in 1984, and due to catch reductions, the Australian Fish Stocks Report for Snapper (2020) Denham Sound (WA), is classified as sustainable.
Peter was asked to visit Florida to look at the fish stock numbers for Scalloped Hammerhead Sharks (threatened) in the Gulf of Mexico. He found multiple independent surveys that were not from fishermen but from research and university institutes. Despite falling stock numbers from 2000 to 2010, the stock size is now increasing, and the current catch is considered sustainable.
Willy Dadour