LGCAA

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Temporal Fish Population Survey Of The River Ivel 2006

1. Introduction
During early April 2006 Central Areas Ecological Appraisal team was due to carry out its routine fish population survey of the River Ivel between Henlow and Tempsford. Unfortunately following concerns from Health and Safety regarding boat stability the survey had to be postponed. These worries proved to be unfounded and the survey was able to proceed in early June, however by this point the usual time scale for the survey had been exceeded by at least a month. It was expected that this variation in time may have led to some degree of degradation in the results collected, but it was decided that it would be better to continue with the survey than to cancel it and lose a concurrent data set.

2. Biological and Chemical Quality
Excellent/Good biological quality (GQA Grade A/B) was noted based on the diversity of organisms living in the river and on the riverbed.
Good/Moderate chemical quality (GQA Grade B/C) was noted based on biochemical oxygen demand, dissolved oxygen and ammonia levels.

3. Survey Sites
A total of 5 sites were sampled by Electric fishing between stop nets.
See map1 for further details.

4. Method
The River Ivel is sampled using standard Environment Agency electric fishing methods, however two boats are used to accommodate the increased channel width. Stop nets across the channel isolate the survey site to prevent migration of stocks into and out of the survey area. The team moves downstream along the channel wielding anodes connected to an electrofishing box set to output Pulse Direct Current (PDC) and selected voltage through the anode ring. A multi tailed, or large surface area cathode completes the circuit. Fish within the field of influence are exposed to increasing effects of the current the closer to the anode ring until galvanotaxis is induced into the fish, which inhibits primary motor functions such as swimming, and the fish is captured. The process occurs twice to provide reliable quantitative data on which a population estimate can be based. All fish caught are measured while sub samples have scales removed for subsequent age determination.

5. Results
Results are given for fish >99mm.
Electric fishing sites and respective biomass categories are expressed as map 1.
Biomass and density estimates are presented as tables 1 and 1a for biomass and tables 2 and 2a for density.
A mean biomass of 27.9gm^-2 and mean density of 0.09 fish/m^-2 was recorded.
A total of 11 species were recorded.
Chub dominate by biomass (18.4 gm^-2) and by density. Pike were subdominant by biomass and Roach by density.
The greatest biomass recorded in 2006 was found at site 335 Henlow with 61.5 gm^-2 noted.
The highest density of 0.25 fish/m^-2 was recorded at site 362 Gt.Ouse Confluence at Tempsford.

Chub
Chub formed the dominant component of biomass contributing 66% of the overall weigh recorded at 18.4gm^-2 representing a slight decrease since 2005 when a mean biomass for the species of 22gm^-2 was noted.
Mean Chub density remains comparable of 0.02 fish/m^-2.
Chub were present at all of the survey sites and were most abundant at Henlow (site 355).

Pike
Pike contribute 15% to the overall biomass. Recorded Pike biomass has decreased by 3.5gm^-2 since the last survey.
Pike density remains low at <0.01 fish/m^-2.
Pike biomass was highest at site 358, downstream of the Iron Bridge at 5.3 gm^-2.

Perch
Perch were third dominant species by biomass and were found at all sites. Perch contributed 7% to the total biomass.
Mean biomass for the species remains comparable since 2005 at 2.1gm^-2.
The greatest weight of Perch (4.6gm^-2) was located at Henlow with individuals to 36cm recorded.

Barbel
Barbel were found at three of the five sites. Barbel biomass appears to have decreased to 4.4gm^-2 with a mean of 0.7gm^-2 recorded.
As expected the most prolific area was site 358, downstream of the Iron Bridge, where a number of mature species were recorded.
Juvenile Barbel were recorded at three sites with site 381, Girtford Weir producing the highest density of immature fish.

Other Species
Roach biomass and density remains comparable to the previous survey, while Dace biomass and density has shown an improvement, particularly at site 362 Gt.Ouse Confluence.
Despite being found at only one site, Common Bream biomass has also shown an increase thanks to an improved catch on Biggleswade Common.

6. Discussion
The River Ivel Henlow to Tempsford is currently Class A: Excellent fishery with a mean biomass of 28.3gm^-2 and density of 0.15 fish/m^-2. The current survey however appears to show a decrease in both biomass and density when compared to the 2005 data set.
Examination of the following graph illustrates the fluctuations recorded in biomass and density over the past 6 surveys and helps show how the capture of a large aggregation of fish can make subsequent survey years appear comparatively poor. The influence of a large Bream shoal captured at Girtford Weir in 1998 and exceptional numbers of Chub and Barbel at the Iron Bridge site in 2005 show clearly as exceptional peaks on the graph.

Although site 358, downstream of the Iron Bridge, retains a Class A: Excellent biomass category a major part of apparent decline in 2006 has occurred in this area. Recorded biomass has dropped by 48gm^-2 as numbers and weights of Barbel and Chub captured here failed to show in 2006. While this seemingly dramatic loss to the system may cause initial concern, a special survey specifically planned to examine Chub and Barbel populations between Biggleswade Mill and Common had occurred earlier in the year and appeared to indicate that the bulk of Barbel and Chub stocks were currently inhabiting the section upstream of the Iron Bridge. This appears to confirm Barbel and Chub biomass has simply moved rather than having been lost from the system through mortality, natural or otherwise. Site 355, Henlow, has shown a massive increase in biomass with the recorded bulk of fish doubling since the previous survey. Chub dominated with 118 individuals recorded to specimen size and are obviously exploiting suitable habitat provided by the riffle structures positioned by the EA. Site 361, downstream of Girtford Weir showed a slight decline in biomass due to loss of Pike and Chub however increased numbers of Roach and Dace at this site helped to offset this. Mature Bream have not been recorded at Girtford for the past two years. When reasonable numbers were last recorded in 2003 they were aged at between 12 and 19 years. As the average life expectancy of the species is 20 years it is likely that there are few mature adults left in this section, and lack of juveniles suggest that poor spawning success will have further impacted on Common Bream populations in the vicinity.

7. Conclusions And Further Management
Reductions in Chub and Barbel biomass are likely to have been caused by stock migration rather than mortality. The works at Henlow continue to provide extremely suitable habitat, which is being utilised by substantial fish stock. The area should benifit further from the addition of a gravel riffle by the Brampton Fisheries, Recreation and Bio-diversity team. The availability of habitat is likely to be a limiting factor for population expansion, particularly with Barbel, downstream of the Iron Bridge section where the channel deepens and slows. To address this, one pair of boulder croys have been installed and a further two more pairs are planned to increase diversity of channel structure. Native trees will also be planted to provide shading and cover. The River Ivel remains a class A biomass fishery and should continue to offer excellent sport, particularly for specimen anglers seeking Chub and Barbel.