Last week’s Central Abaco Lionfish Derby was another success (LINK)! At the end of the tournament lionfish were stacked up for counting and measuring – over 1060 fish in one day! As we looked at all those fish, many wondered – Are the returning boats bringing fewer than in previous years? And, are the ones caught smaller or larger than in previous years?
My hope here is that I can help answer some of those questions using data from previous Central Abaco Derbies. Thank you to Friends of the Environment for sharing these data so that I could try to address these questions.
First, let’s first make our questions as explicit as possible, to avoid any confusion:
- Q1 – Are fewer lionfish being brought back to weigh-in over the years?
- Q2 – Is the size of the largest fish per catch changing since the start of the derby?
But before we dig in, let’s cover some basics: The data I looked at includes all of the returns – that is, all the data from years 2010, 2011, 2012, 2014, and 2015 (the 2013 derby was cancelled because of an unfortunate fire in the Fish House). A total of 31 different ‘boats’ participated in the Derby across that time. By ‘boat’ I mean the name of the boat entered into the competition. Several boats competed in multiple years (something I will look at later in the post).
Let’s start with Q1: “Are fewer lionfish being brought back to weigh-in over the years?”
A crude analysis of the number of fish per boat (43 data points) shows that there is little-to-no change since the start of the derby. Lionfish brought back per boat varies widely from a high of 622 to a low of 0. But there is no appreciable trend over time suggesting that the number of fish per boat has not changed from an average of 154 since 2010 (but read on).
*Change over time was tested by regressing fish per boat over time (year). The effect of year was not significant (p = 0.56)
Now on to Q2: “Is the size of the largest fish per catch changing since the start of the derby?”
Another crude analysis shows that the size of the largest lionfish caught per boat has not changed either. The largest fish caught over the time period was 40 cm. However, the average size of the largest caught per boat is about 30.6 cm and that does not show any trend since the start of the Derby.
I also looked for change in the size of the largest fish overall brought back per year. I also found little evidence for change over time. That said, I will note that there does seem to be a trend is for the largest lionfish caught per year to increase slightly, but again I don’t quite trust these data yet. Maybe in a few years I will be able to tell with confidence.
*Because the size of the largest fish caught per boat is correlated with the number of fish caught, the number of fish caught was included as a covariate in the statistical model.
So far, it looks like we don’t have any good evidence for change in the number of fish per boat or the size of the largest fish brought in. Boats bring in 150 fish on average, and the largest fish per boat is about 30 cm.
However, let’s dig a little deeper. Lets focus on boats that participated in multiple years. There are six boats that were involved with the derby for more than 1 year since 2010. Some of these boats only repeated once while some were in it for 4 years. Assuming that these boats contain roughly the same fishermen, exert similar effort among years, and fish the same areas, I thought that asking the same question of this subset might shed a little more light on trends over the derby period.
Lets ask Q1 again using the repeat subset: “Are fewer lionfish being brought back to weigh-in over the years?”
These results differ quite a bit from our conclusions reached above. In fact, these data show a roughly 50% decrease in the number of fish per boat since 2010. These estimates are rough – and I wouldn’t put too much trust in them – but they certainly give the impression that the number of fish per boat has declined substantially. For example, between 2011 and 2015 Kiki’s returns have dropped steadily from 514 to 309. This is not the rule though as several boats showed no change and one seemed to increase returns since 2010. Nevertheless, this analysis show that the trend is decreasing as can be seen in Figure 3.
*For the stats nerds, these data were analyzed with a linear mixed model in which totals were tested against year with boat included as a random term. The plotted data are totals standardized by boat over year. The linear mixed model term ‘year’ was significant (p = 0.02).
Ok, now lets look at Q2 again: “Is the size of the largest fish per boat changing since the start of the derby?”
Again, there was no trend. The size of the largest fish caught per boat has stayed about 30 cm since the derby began. This is the same result that we saw when we look at the entire dataset. Remember that the numbers on the vertical axis do not correspond to the actual size of the fish as they have been statistically standardized to show the trend (or lack of one) over the derby period.
*the p-value for the year variable was 0.43 for this analysis.
Well, from the Central Abaco Derby data it looks like there might be a decrease in the number of lionfish brought in since 2010. However, there does not seem to be a change in the maximum size of the fish. But the major thing we all want to know is whether there are fewer lionfish out there in the sea. It’s difficult to translate Derby data into population data, but by talking to folks that spend a lot more time in the water than I do, it seems that there are fewer lionfish out on the reef. This agrees pretty well with a lot of the anecdotal data we have coming in from many different sources (but see link). Perhaps the lionfish derby offers a way to keep track of the size of the lionfish population over time?
What is causing the decrease? I don’t know. There are many questions that still need to be answered, but it’s clear that the lionfish derby is an important commnity activity. Opportunities to collect data at such a large scale over multiple years is a difficult and expensive endeavor (participants in the Central Abaco Lionfish Derby typically fish from Green turtle down to Little Harbour – about 75 km of coastline) and therefore serve a valuable ecological service by: 1) helping to reduce the number of lionfish, and 2) collecting important data on lionfish population size. We’ll see what future Derby’s will show!
Thanks to sponsors, participants, volunteers and to Friends of the Environment for keeping it going!