Question: what fish, and when?

What fish do the Rutland Ospreys catch in the reservoir?

And when do they usually go fishing?

In 2004, incubation lasted 38 days and the monitoring team recorded a total of 90 fish being brought in by the male for the two adult birds to eat. Tim Mackrill analysed these data and two surprising (to us!) results emerged. They concern what fish were brought in when. Firstly there is a very high proportion of roach and far fewer trout than we expected. Rutland Water is stocked with trout, but roach must be much easier for the male to catch at this time of the year. Secondly we were under the impression that Rutland Ospreys are easiest to see fishing in the early morning. We expected that another popular time was the evening after a quiet spell in the middle of the day but we were very surprised that so many fish (nearly 40%) were brought to the nest between 4 and 8 pm.

The male delivering a roach to the nest. Photo: John Wright

Species of fish caught by 03(97) during the 38 days of incubation in 2004

Number of fish caught in hourly intervals during the 38 days of incubation in 2004

We invited email comments and suggestions about this and the following responses were received.

Neil Kilburn said:

As a fellow angler (to the Ospreys that is) I think that I might be able to put some ideas as to why they catch more Roach and other coarse fish than Trout.

1. Coarse fish in Rutland water and the nearby lagoons are far more numerous than the Trout stocked by Anglian Water and therefore would naturally make up more of the Ospreys diet.2. Trout are designed to eat flies and other water invertebrates that they can see above them and so they have very good upward vision Roach do not.3. Roach as the summer progresses will spend more time at the surface basking in the sun, whereas the Trout will move into deeper water where the temperature is lower.4. Trout are better camouflaged than Roach, Rudd, Perch etc.

5. Roach are slower than Trout and easier to catch.

Phiip Tew said:

The main reason why less trout are caught than any other fish is due to changes in stocking policy which has resulted in bigger fish being stocked in order to reduce losses to cormorants! ( this in turn has resulted in fewer cormorants at the Nature Reserve!

The reason why more fish are caught between 4 & 8 pm is almost certainly due to the behaviour patterns of the fish food supply, which will predominantly be insects in one form or another. Generally these become quite active in the early mornings and late afternoons, which in turn bring fish to the surface to feed, and as we know Ospreys rarely if ever catch fish below a depth of 1 metre. It could be as simple as this !

Pete Barron of the Lake District Osprey project wrote:

Although I do not yet have this years data to hand, here is a lifted extract from last year's info which you may find of interest as it is very similar to Tim's findings:

25  (28.09%) of these fish have been between the hours of 4am and 12 Midday

64 (71.91%) have been between Midday and 10pm

Apart from 8 to 9am (8 fish) the most productive times are from 4pm to 10pm with 44 fish (49.44%) brought in

I have always assumed that, as in Scotland, that trout would be caught later in the season as they tend to rise to the various fly hatches which are not as available, certainly further north, in the early season. Pike and other spawning fish such as roach and perch tend to come into more shallow water to spawn earlier in the season from March to June. Could this be the reason for more roach in the diet than you expected?

It will be interesting to see if successive years show a similar pattern (first observations are that they do).

Andy Stafford wrote:

1. Roach are a better size for an osprey to catch as trout are stocked into the reservoir at (generally) 2 pounds or greater. The roach average rather less, generally in the half to one pound range. Much safer and easier for the osprey to catch and carry off. Less chance of getting dragged around and saturated by a roach than a trout. Much more chance of a trout pulling itself free from the claws too.

2. Afternoon will be a favourite feeding time because that is when many of the reservoir's fish will be closest to the surface and therefore most accessible to an osprey. The fish will be at the top
a) because they like the warmer water (don't we all?),
b) most insect hatches will occur as the water warms during the day, so during the late afternoon many emerging flies will be in the upper layers, pursued by the fish,
c) if there is a decent ripple on the water and as long as it's not too hot, then the upper layers will tend to be the most oxygenated which the fish can also find attractive.3. So far as species are concerned, I know there are a few chub in the reservoir. It's possible that these make up some of the "unidentified" species. Because of the shape of them, they can easily look like other species too. (Especially when in pieces!)

4. Finally and most obvious, the chicks are eating very strongly in order to build body mass and strength for flying/migration. With the parents presumably unable to hunt during dark, I guess that the parents are taking the opportunity to "stock them up" in the evening for the "fast" during the dark hours.