Algae-eating luderick in marine parks

[Australia] There are only a handful of primarily herbivorous fish species commonly targeted by anglers in Australia. Of these the luderick, or blackfish (Girella tricuspidata), is probably the most widely recognised, and certainly is targeted more often in our east coast estuaries than most any other herbivore around these parts. Indeed, this species is most commonly targeted in the estuaries and coastal waters of Australia’s east coast south from around Hervey Bay to Victoria, but they also occur as far west as near Adelaide, including most areas of Tasmania, as well as the northern parts of New Zealand, where they are known as Parore.

A number of the luderick’s close relatives in the family Girellidae are less commonly targeted around Australia’s southern coastline, including black drummer (Girella elevata), the zebra fish (Girella zebra), and WA’s rock blackfish (Girella tephraeops). Though often described as herbivores, all of the above species are probably more accurately described as omnivores. Much of their natural diet consists of a few different types of algae, but they also eat quite a lot of other animal material including small crustaceans, molluscs, and polychaetes. This is where the members of the family Girellidae differ markedly from members of the closely related family Kyphosidae, which includes the silver drummer (Kyphosus sydneyanus), which use microbial fermentation and other specialist modifications in their guts which allow them to eat algae almost exclusively and can rightly be described as true herbivores.

With the recent proliferation of marine parks (MPAs) in Australia, there has been increased interest in examining the role herbivores like luderick and drummers play in the ecology of temperate rocky reefs. Conventional tagging studies of over 6000 luderick by fisheries scientists over many years showed that while most short term (less than 12 months) recaptures show little movement, for fish tagged for longer time periods a significant proportion of the population undertake migrations (mainly northward) up to 455 km (average 160 km) from their point of capture, probably associated with spawning behaviour. In contrast, recent electronic tagging studies conducted in the Jervis Bay Marine Park have found the majority of the small number (6) of luderick tagged with electronic tags stuck to relatively small home ranges, at least over a short time scale of 11 months. Battery failure was implied as the reason why the fish weren’t tracked for longer, however in a preliminary paper the authors claimed the life of the transmitters was 738 days, not the 286 days indicated in the subsequent paper. Reasons for this discrepancy and why the fish were not tracked any longer than 11 months in the MPA were not disclosed.

Because luderick have some site attachment (at least in the short term) and graze on algae, marine parks scientists became interested in them as work on coral reefs in the Caribbean has suggested that herbivorous fish can influence the health of corals by eating algae that otherwise could overgrow coral colonies. However, what is often overlooked by proponents of this theory is algal overgrowth only became a problem in the Caribbean after a massive die-off of sea urchins in 1983-4 due to an outbreak of disease. Turns out the sea urchins are keystone species which control algal abundance, and there is now evidence that corals on the affected coral reefs recover anyway once sea urchin populations recover, irrespective of whether fishing for herbivorous fish occurs or not. Regardless of this, protection of herbivorous fish for their theoretical ecological benefits remains one of the key reasons cited by MPA advocates for establishment of green zones in not only coral reefs, but now more recently in temperate rocky reef ecosystems.

To try and work out if luderick and other herbivorous fish contribute to the health and ecological functioning of rocky reefs in Jervis Bay, scientists examined if establishment of green zones boosted luderick numbers and influenced the ecology of the areas they frequent. These recently published studies make for interesting reading. The authors found that luderick were “significantly larger and more abundant” within green zones and claimed that this may benefit luderick fisheries and the rocky reef ecosystem. Sounds impressive, but what does this actually mean ?

Closer scrutiny of their data showed that the average number of luderick per 50 metre snorkelling transect was 8.8 fish in green zones compared to 4.7 in fished zones. At face value this suggests that fishing might impact luderick numbers in Jervis Bay, but without baseline studies pre-zoning, it’s impossible to tell for sure as site selection could also play a significant part in this result. Also, recent research by University of Queensland scientists found potential errors with visual fish counting methods caused by different fish behaviour between fishing and non fishing zones. It appears that fish in areas that are fished are more cautious and less likely to closely approach baited cameras (or divers for that matter) so are less likely to be counted compared to fish in green zones. Interesting !

In the Jervis Bay luderick study, much emphasis was placed on the larger average size of luderick within the green zones (mean 29.6 cm), but those outside the green zones were only 1 cm smaller on average (28.5 cm). Given none of the fish were actually measured directly (instead using a video system with an accuracy of within 0.5 cm), even if such a tiny size difference was statistically significant, are the size differences biologically and ecologically relevant ? Especially given that we already know the 27 cm minimum size for luderick in NSW is still around 3 cm or so less than what fisheries scientists recommend to allow more fish to spawn before harvesting? Surely we’d want an average size well above 30cm to provide a significant potential boost to reproductive output, and where is the evidence luderick spawn in those green zones anyway? (p.s., there is none).

Despite admitting that no baseline surveys were conducted prior to establishment of the MPA, the scientists in the Jervis Bay study stated their results suggest that green zones “worked” and should “lead to significant increases in the size and abundance of luderick within marine reserves over time, given the rate of fishing mortality is greater than that of natural mortality”. Strong claims, given they can’t prove these fish stay in green zones for longer than 11 months. Furthermore, the green zones were established back in 2002 so despite the lack of baseline data, during the study fishing restrictions had already been in place for around a decade. Given that luderick mature in 4-6 years, 10 year old luderick are 35-40cm long, and this species has a maximum age of around 25 years, it is reasonable for local fishers to ask how much longer will they have to wait before any biologically meaningful increases in sizes of luderick inside green zones are found?

Given there was approximately double the number of luderick counted on average in the green zone, the scientists suggested this should “potentially lead to measurable differences between reserves and fished areas, in terms of algal community structure, over time”. They also concluded their findings “demonstrate the clear potential for greater grazing by herbivores within temperate marine reserves”. This all sounds logical at face value, as more luderick should eat more algae, but that logic firstly assumes luderick will remain in the green zone long term, which was not proven by the short term electronic tagging of a small number of fish.

Indeed, that assumption may be disproven by the much larger database from long term conventional tagging studies. They also assumed that fishing was responsible for increased numbers of luderick in the green zones, but this assumption is confounded by the lack of baseline data, potential sampling bias (as shown by the UQ researchers), and the results of another study published by the same group in 2015 that showed luderick abundance on rocky reefs was seasonally related to food availability and the largest schools of luderick were found on reefs with the highest cover of sea cabbage (Ulva spp.). Over the entire year, however, luderick numbers bore little if any relationship to algal abundance, perhaps because luderick can switch to other food items seasonally or when suitable algae are scarce. In any case, their data shows if you want to find areas with more algae, look for the areas with more luderick, which is logical if you are a hungry luderick, but the opposite of the usual “herbivores control algae in MPAs” dogma.

In the face of this apparent data conflict, the hypothesis that herbivorous fish like luderick regulate algal growth and ecological function on temperate rocky reefs in NSW appears to be on very shaky ground. This actually makes biological sense if luderick are in fact not a keystone species, unlike known ecosystem engineers such as sea urchins. So, after a decade of waiting, there appears to be little to be gleaned from these studies of herbivorous fish in NSW MPAs except for the certain fact that green zones stop fishing. Any other potential benefits remain just that – potential and still as yet unmeasured. What do you think? After a decade should more be expected from marine parks science than conclusions that management success has been achieved by stopping fishing in green zones ?

 

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