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Ask Dr Dazza | Identifying the Grass Sweetlip fish

Posted: 12pm 10 Mar 2022

Moreton Bay has a wonderfully diverse array of fish for the recreational angler to catch. There is just one problem with this for the average angler and this is correctly identifying them.

Why does correctly identifying them matter? Because different but similar looking and related fish can have different size and bag limits. The reason for this difference is that they can have vastly different growth rates and size at maturity which necessitates them in many instances.

The first rule of keeping a fish is that you need to know exactly what it is. Not possibly know – know with complete certainty. Not only does this avoid breaking the size and bag limit rules, but it also avoids possibly consuming something that is poisonous or potentially poisonous. If your catch is inspected by compliance officers or you post a photo to social media of an undersized fish, not knowing the rules is not a valid defence.

Once upon a time Grant’s Guide to Fishes was THE bible for any keen fisher to identify a fish and while it remains a go to publication, there are many apps now available. The State Government has the free FishSmart app which provides information on size and bag limits, fish identification and other information to help fishers comply with rules.

In this column and the next I am going to go over some of the species that frequently confuse recreational anglers in the Moreton Bay Region. Let’s start with the grass sweetlip or grassies as they are called and spangled emperor.

The grass sweetlip (Lethrinus laticaudis) is a common reef fish caught in Moreton Bay including around the reefs of Woody Point and Scarborough. It has a minimum legal size of 30cm and a bag limit of 10 fish. For management purposes it is identified as a rocky reef fish species. While always present in Moreton Bay, spangled emperor (Lethrinus nebulosus) appears to be becoming more widespread including on the western side of Moreton Bay at places like Redcliffe and Scarborough. It has a minimum legal size of 45cm and a bag limit of five fish and for management purposes is identified as a coral reef fish species. The grass sweetlip does though remain the most common type of sweetlip caught inshore in the Moreton Bay Region.

The trap for the average angler is that they are frequently confused. I have seen more than one angler display a dead undersized spangled emperor on social media thinking that what they had kept was a legal grass sweetlip.

While grass sweetlip tend to be shades of tan or brown and spangled emperor shades of olive, depending on the habitat, size of fish, time of day and the individual fish these general colourations may differ. Thus, general overall colouration is not the most reliable method for differentiating between the two species.

To identify which species you have, the first thing to look at is the scales. The individual scales of spangled emperor have blue flecks on their edges which form the blue streaks along their flanks, while the individual scales of grass sweetlip have black markings.

The second thing is that spangled emperor have three relatively long blue streaks or series of dots that extend from the eye across their snout, while grass sweetlip have a series of much shorter and narrower blue lines that radiate from their eye.

The third thing is that there are subtle differences in body shape that become clearer if you put the two fish side by side or see enough of both over time. Spangled emperor tend to have narrower bodies and a slightly more elongate snout area compared to grass sweetlip.

Tight lines everyone, but make sure you know what was on your line before you keep it!

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