King mackerel otolith shape analysis

King mackerel, Scomberomorus cavalla, are biological torpedoes, sleek, long, silver bullets that live in nearshore waters of the Gulf of Mexico and US Atlantic coast. They are a popular gamefish throughout their range, known for making impressive first strikes on fish or squid lures. “Smokers”, they are called, for what happens to the gears inside fishing reels when a big one hits. They are feisty fish, and big too, with an IGFA world record of 42 kilos (93 pounds). They are important for recreational fisheries in the southern US, and they also support a substantial commercial fishery.

Management of king mackerel is a bit complicated. First, they are found across two management zones, one in the Gulf and a second in the Atlantic. Second, they are seasonal migrators – they move south when water temperatures cool during fall/winter and north as water temperatures warm again during spring/summer. During winter, both Atlantic and Gulf migratory groups are found in south Florida where they mix around the Florida peninsula. Problematically, they both experience recreational and commercial fishing when they are mixed up. So, what is the effect of these mixed-up fish around Florida for the overall management of the species? It would be helpful if we could determine how many fish from this mixed region were of Gulf and Atlantic migratory-group origin.

Welcome to my Master’s thesis and my first publication, Clardy et al. 2008!

My mission during my thesis was to perform otolith shape analysis on Gulf and Atlantic king mackerel during summer when they are separate, and then do the same for fish collected in winter around the Florida peninsula. The overall goal was to estimate the proportion of Gulf and Atlantic migratory groups to the winter catch in three zones around the Florida peninsula so that management of the species could be improved. Up to that point, the fisheries management approach was a bit arbitrary in assigning all fish from south Florida to either the Gulf or Atlantic depending on the time of the year. If I could determine the pattern of how Gulf and Atlantic fish mixed up in winter, then managers could have a better idea of what to do with the fish caught in south Florida.

Wait. What’s an otolith? And what does their shape have to do with anything?

Otoliths are structures in the inner ear of bony fishes that the fish use for balance, hearing, and orientation. They are composed of calcium carbonate and continuously grow throughout the life of the fish. They wind up being super important for fisheries scientists for a variety of reasons, primarily as a way to age fish. It also turns out that otolith shape is species specific. So, the shape of a king mackerel otolith is different from that of its close relative, the Spanish mackerel, Scomberomorus maculatus. Even better, the shape of otoliths within king mackerel is different between Gulf and Atlantic groups, because Gulf and Atlantic fish have different growth rates. Good news for me.

I performed shape analysis on otoliths from over 1100 mackerel collected from the Gulf and Atlantic. I used a suite of shape analysis metrics, everything from otolith length and width to Fourier analysis. I broke down the fish by year, region, time of year, sex, age, and even left vs right otoliths. Once I calculated the shape of Gulf and Atlantic king mackerel otoliths from summer samples, I broke Florida into three regions and calculated the origin of winter-caught fish around the Florida peninsula.

This analysis was before the days of R, so it was done with a combination of Image Pro, SAS, and a customized program in S-Plus. I melted down a computer estimating the proportion of Gulf and Atlantic fish around Florida in winter. I had to run part of the analysis in batches due to RAM issues; if I tried to run all the data at once, the RAM allocated to S-Plus would fill up, and the computer would crash. I melted down my personal laptop running the analysis due to overheating and had to borrow the lab laptop to finish up.

At the end of the analyses, I had a pretty good estimate of mixing of king mackerel around the Florida peninsula during winter. On the Gulf side of the peninsula, 2/3 of the fish were from the Gulf, at the southern tip of the peninsula, the breakdown was 50/50 Gulf/Atlantic, and on the Atlantic side of the peninsula, 2/3 of the fish were from the Atlantic. These results are a bit intuitive if you think about it. The cool thing was that I was able to put numbers on the estimates and hand that off to the king mackerel management council. It’s the equivalent of showing your work in math class. We published a peer-reviewed manuscript that shows all the gritty details, so managers have a clearer understanding of a complex issue.

King mackerel management is one of the great success stories in US fisheries management. Things were going pretty well before I came along. Catches for both recreational and commercial fishing are in good, stable shape and have been for a few decades. I’m happy that my thesis helped improve management going forward so that the future of king mackerel fishing remains bright.

Have you ever tangled with a Smoker king? Want to know more about otoliths? Drop a comment below, via email at [email protected], or on Twitter!