Fish Facts: Estuary perch

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One way to distinguish an EP from a bass is by the concave shape of the head. It isn't always as distinct as it is in this pic though! Image: Scott Thomas
One way to distinguish an EP from a bass is by the concave shape of the head. It isn't always as distinct as it is in this pic though! Image: Scott Thomas

ESTUARY perch (Macquaria colonorum) are perhaps Australia's most enigmatic estuarine sportfish. Like its closest relative the Australian bass (Macquaria novemaculeata), estuary perch (or EPs) were historically abundant in major coastal drainages from northern NSW to the Murray River in SA, as well as a few rivers in northern Tasmania. But since the mid 20th century their populations have declined (in many cases, precipitously) due to historic commercial fishing as well as more recent habitat changes since these catchments were cleared for agriculture and urban development.

Bass and EPs are very closely related, and because of this they are similar in appearance. Indeed in the past some fishers considered them as one and the same species. Aussie sportfishers who caught both species on lures referred to them all as bass, possibly inspired by the lurefishing literature that referred to their US equivalent the largemouth bass. The bait soakers, on the other hand, (who incidentally caught the majority of these fish on baits like fish, worms, molluscs and shrimp), referred to them all as "perch". Without having both species in hand at the same time, it can be difficult to tell them apart, but the EPs have a distinctive concave profile to the head (convex in bass), a more compressed (deeper and narrower) body profile, their lower jaw protrudes markedly further forwards compared to bass, as well as other detail features. EPs also grow to a larger size, having been recorded to over 75 cm long and 10 kg. Growth is rapid until fish begin to mature at 22-25 cm at about 3 to 4 years of age, after which it slows dramatically. Mature fish around 8-10 years old can be around 26-36 cm long, while 15-20 year old fish may range between 28 and 48 cm. The oldest EP aged to date was a 40cm fish aged at 41 years, and there is a strong trend whereby the larger fish tend to be mainly females.

To make their identification even more difficult, both species are not only similar in colour (olive brown above to silvery white underneath), they can also hybridise. Scientists have confirmed that Australian bass and EPs naturally cross breed in several of the more southern rivers where their distributions overlap (from southern NSW to the Gippsland Lakes in Victoria), producing hybrids with intermediate morphological characteristics. In the past it was considered that in river systems containing both bass and EPs, their distributions seldom overlapped because EPs live their entire lives in brackish or tidal reaches of rivers and coastal lakes, usually in the deeper (over 5 meters) areas in the middle reaches of the estuary. In contrast, bass live mostly in the shallower (less than 2 metres) freshwater upper reaches. But adult bass migrate to the estuary in winter to spawn, and in cases where hybridisation has been recorded, the hybridisation was due to female bass breeding with male EPs. It seems that this occurs most often in rivers where bass are very uncommon and outnumbered by EPs. Usually the two species breed at slightly different times of the year (bass earlier than EPs), at different salinities in different parts of the river system. However, under some circumstances, running ripe male EPs can encounter late migrating female bass, thus presenting the opportunity for hybridisation.

Recent acoustic tagging studies of EPs have provided some fascinating insights into exactly what these fish get up to. These studies proved that EPs predominantly reside in deep pools in areas containing “structurally complex large wooden debris” (i.e. snags) for most of the year. However, during the winter-spring spawning period, mature EPs would move downstream up to 50 km to the lower reaches of the river, and aggregate around specific snags in close proximity to the river entrance. Some fish would repeat the spawning migration several times per season, particularly during wet periods with higher river flows. Peaks in EP spawning occurred slightly earlier in the year (June-August) in more northern rivers like the Hawkesbury and Clyde, compared with the rivers further south in Victoria (September-November).Plankton tows confirmed that spawning events coincided with these aggregations around particular snags, with egg abundances peaking at night during the first two hours of the run-out tide. It is thought this behaviour facilitates dispersal of eggs and larvae into coastal waters, allowing connectivity between different estuaries. Indeed, larvae of both bass and EPs have been collected as they re-enter other estuaries from the ocean on flood tides.

This reliance on particular “spawning snags” by EPs is thought to be a potential bottleneck in their life history. In many river systems where EPs were formerly abundant, degradation of riverbank habitat and removal of riparian vegetation has reduced availability of large wooden debris, potentially limiting EP spawning output by limiting the number of fish that can physically fit into the area of preferred habitat present within a spawning ground. This spawning aggregation behaviour also makes EPs vulnerable to fishing. The best EP fishing no doubt occurs when anglers locate the spawning snags and target these pre-spawning and spawning aggregations. Because of this, obviously it’s important to observe closed seasons when they occur and try not to interrupt spawning fish. At other times, catch and release is obviously recommended to try to conserve the numbers of large female spawners. Indeed, even though EPs are not allowed to be taken by commercial fishers, there may be significant unrecorded netting bycatch, as recent research indicated that larger, older (over 10 years) EPs were very rare or absent in river systems that were subject to commercial fishing.

Recent research also suggests that recruitment of young EPs is infrequent, and that because of their extreme longevity, many EP populations are currently being supported by fish that were spawned 15, 20 or even 30 years ago. Because detrimental changes to their habitat has no doubt occurred in the last 30 years in many coastal catchments around our coastline where urbanization is relentless, this means that management of healthy populations of EPs is likely to become increasingly difficult into the future. Its clearly up to recreational anglers to work on both habitat enhancement initiatives, as well as adoption of predominantly catch and release fishing, if wild populations of EPs are to remain available and sustainably fishable for future generations.  

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