Marohasy on cod
I’ve had an interesting discussion over at the new blog of Jennifer Marohasy, who runs the environment unit at the Institute of Public Affairs. Marohasy has criticised virtually all the main scientific groups working on the Murray-Darling Basin, and in this post, she nominates The NSW Rivers Survey by the CRC for Freshwater Ecology and NSW Fisheries. as the “worst ever”. She says
The report’s principal conclusions include that “A telling indication of the condition of rivers in the Murray region was the fact that, despite intensive fishing with the most efficient types of sampling gear for a total of 220 person-days over a two-year period in 20 randomly chosen Murray-region sites, not a single Murray cod or freshwater catfish was caught.”
Most remarkably at the same time, in the same years and regions, that the scientists were undertaking their now much-quoted survey that found no Murray cod, commercial fishermen harvested 26 tonnes of Murray cod!
Criticism of the report’s findings from a local fisherman goes something along the lines “The scientists, although having letters behind their name, spending some $2million on gear, and 2 years trying, evidently still can’t fish.”
Zing! Those egghead scientists are conclusively nailed! Well, not quite.
Going to the sentence cited by Marohasy, we find that it doesn’t end where she stops the quote. The actual sentence reads (emphasis added)
A telling indication of the condition â€¨of rivers in the Murray region was the fact that, despite intensive fishing with the most efficient types of sampling gear for a total of 220 person-days over a two-year period in 20 randomly chosen Murray-region sites, not a single Murray cod or freshwater catfish was caught, although more than 50 of each species were found at Darling-region sites.
It looks like the scientists can catch fish after all.
Still, you might say, it was naughty of them to imply that there were no Murray cod left in the river. Except that they didn’t. Immediately following the sentence cited by Marohasy, the report goes on
While it is well known that Murray cod do remain in some parts of the Murray region which were not sampled in the Rivers Survey, and even continue to support fisheries in some areas, the results emphasise that the populations of this keystone species are now fragmented and patchy, and their overall abundance is worryingly low. Relatively high catches of Murray cod are taken by anglers and commercial fishers targeting remnant populations in some key Murray-region habitats, but these fishery catches clearly do not reflect the true condition of the population overall.
After a bit of back and forth, Marohasy backed away from the suggestion that there were plenty of cod in the sites sampled by the study and said
One of the main reason the scientist caught no fish is that they broke some of the basic rules of samplying animal populations; the rules you used to learn in 101 Zoology including do not assumed a random distribution.
Except that as we’ve already seen, it’s not true that the scientists didn’t catch any fish. They caught other species, but no Murray cod, in the Murray sites.
If the survey had been designed specifically to look for Murray cod, Marohasy’s point would have some validity, though it’s expressed rather sloppily: to put it more precisely, the most efficient estimate in situations of this kind is obtained by stratified sampling, which usually implies focusing on sites where mean abundance is high, since mean and variance usually rise together. But this was a general survey of all species, so the sampling procedure used here was entirely sensible.
While we’re on the subject of 101-level errors, Marohasy commits a big one herself, in her IPA paper on which the post is based. Most studies have taken the collapse in the commercial catch, illustrated below (source), as evidence that cod numbers have declined.
Marohasy notes that as catches have declined so have the number of fishers (partly voluntary exit and more recently progressive closure of the fishery). She suggests a better measure would be catch per unit of fishing of effort, which rose as numbers of fishers were cut.
But a course in Fisheries Economics 101 would have told her that’s exactly what you expect in this situation. Overfished stocks show diminishing marginal returns to effort, so a reduction in the number of licenses will produce an increase in average return to effort even if stocks are still declining. Marohasy isn’t an economist of course, but she ought to have checked on a basic point like this.
The real problem, though, is the doctored quote. If you want to attack the professional competence of a scientific study, let alone call it the “worst ever”, you should make sure your quotes are accurate and complete. Marohasy owes the authors of this study a big apology.