Why are mullet, sea trout and flounder so prevalent in Monty's diet? Are there any environmental factors that affect the "choice" of what fish are caught?

As many of you may know, I have been undertaking a Masters of Research course studying our lovely ospreys.

A few years ago in 2013 Stacey Melia conducted an excellent study asking the question “was there an effect of environmental conditions on Osprey chick diet”. The conclusion of that paper was that tidal depth, wind direction, cloud cover, wind direction, temperature, rainfall, tidal depth and moon phase does not affect Monty’s fishing habits in any significant way.

Using this study as a guide I recognised some questions that were left to be answered. To investigate this I have focused on the three key targets for Monty: flounder, mullet and sea trout.

Time until/from high tide

The X axis shows the number of hours since high tide (0 = high tide, 0.5 is 12 hours). The Y axis shows the number of fish caught at each time interval (time interval beginning with 0.1 for example).

As many fisherpeople know, fish move with the tide in estuary systems, feeding at high tide, and move out again with the low tide back into deeper water, and research has shown this to be likely with mullet and flounder. Both species leave deeper water, entering predictable feeding grounds, advancing with the incoming tide.

Sea trout are similar in that when they migrate towards or away from breeding grounds, they move with the tidal current. As we assume Monty’s key hunting ground is within the estuary limits, this would mean that sea trout enter estuaries on moving tides and not high or low tides.

So what do the results suggest?

This graph (above) appears to match the literature quite nicely, showing the number of fish being returned to the nest increases with an incoming tide, dipping slightly at peak tide, and then decreasing again with an outgoing tide.

What time of day are species of fish caught by Monty?

The work done by Stacey showed us what time of the day Monty was most likely going to bring fish back to the nest. So I decided to break it down further to individual fish species and what time of the day they were being brought back to the nest.

The x axis showing the time interval (hour beginning with 08:00 for example, with the y axis showing the number of each species caught).

The literature shows that sea trout prefer to migrate through estuary systems at night, and sit shallower in the water during the night to avoid warmer temperatures. Flounder also feed at shallower depths and come closer to shore in the dark. This could mean these species would be more available to Monty at night, if only he could hunt in the dark! Unfortunately Monty cannot, so how does he manage to catch them?

The biggest spike in the number of sea trout caught appears very early in the morning, between 4am and 9am, with a second smaller spike between 8 and 11pm.

So it appears that Monty is just able to catch the sea trout as they start to move upstream in the evening, and catch the late stragglers in the morning .

In fact, Monty’s catch chart matches a 1998 study of sea trout movement through the Conwy estuary that suggests peak movement is around midnight, as well as an Environment Agency study of sea trout recorded via radio tags.

A comparison of a 2008 Environment Agency table from a study of salmonid movement in esturaries showing the number of fish passing a radio telemetry receiver in an estuary (each dot representing a fish) and Monty's data.

At mid-morning flounder start to appear more frequently, and by mid-day mullet dominate the osprey diet, likely explained by studies that have shown that light levels do not affect mullet behaviour.

What size are the different species of fish being brought back to the nest by Monty?

The number of fish of different sizes caught between chicks hatching and fledging

During the breeding season I decided to take a sample of 100 fish caught between chicks hatching and chicks fledging to explore if the different species of fish being caught were different sizes.

It would appear they might be, with more mullet caught within the category Large (Grey) and joint first for the number of fish in the category Medium (Orange).

However, the statistical test came back as insignificant based on ~100 fish sampled. This was not unexpected however due to the relatively small sample size.

The takeaway message

It looks like mullet are the prey of choice because they are available throughout the day, visit reliable feeding areas predictably with incoming tides, and possibly because larger specimens are more common or available. Also they are highly calorific.

It is interesting that no mullet were caught in the early morning even though they’re unaffected by time of day. Flounder also only becomes more common after the time the literature suggests they should be becoming less common.

This does suggest that sea trout are favoured at this time over other species, but from this small and simple study this would be quite a leap to come to this conclusion. Further work is needed (and with any luck coming up in the future).

Monty bringing a bass to his hungry family

So we cannot tell if Monty is “deciding” to choose certain species at certain times. But going by the findings here, what we can say is that Monty could be a reliable indicator of when fish are moving through river systems and at what times.

Future work

There is still a lot of data to work through and I am in the process of digitising the data spanning 10 years, so a lot more questions might be answerable in the future.

For example, was there a learning process for young(er) Monty to learn where the fish are at different times of the day? Is this a potential explanation for the proportion of mullet steadily decreasing over the years, as Monty has learnt to take advantage of sea trout? 

Hopefully answers will be available soon.

While this short study will hopefully contribute to my overall degree, this is not the main focus of my study. If anyone is interested in this, here is my (unfinished) research poster.

There will also be a document available outlining the statistical analysis used for the above study, feel free to ask if you would like a read.

As always many thanks to the volunteers and staff at DOP for access to the fish records, especially Vicky who also assisted with sourcing some of the litterature used in the above blog.