[MCN] On keeping a river together: 2 takes, same topic

[Lance Olsen]

1-Nunn AD, Copp GH, Vilizzi L, Carter MG. Seasonal and diel patterns 
in the migrations of fishes between a river and a floodplain 
tributary. Ecology of Freshwater Fish 2010: 19: 153-162.

Abstract - The population behaviours associated with the migrations 
of fishes in lowland river ecosystems are amongst the most 
poorly-understood dispersal mechanisms of temperate freshwater 
organisms. This study evaluated the influence of four environmental 
variables (light levels, river discharge, water temperature and water 
velocity) on the timing, intensity and direction of fish movements 
between the River Avon (Hampshire, England) and a small floodplain 
tributary, Ibsley Brook, over a 12-month period. Using canonical 
correspondence analysis (CCA) to identify patterns of movement (by 
groups of species) and the relative strengths of explanatory 
variables in the data, the probability of fishes migrating between 
the river and tributary was determined using Bayesian modelling. The 
intensity and direction of fish movements between the river and 
tributary varied temporally, both on a diel and seasonal basis, and 
there were species- and age-specific patterns in behaviour. Diel 
movements appeared to be triggered by changes in light intensity and 
brook water velocity, whereas seasonal movements were mostly driven 
by changes in river discharge and water temperature, particularly 
those associated with floods. This study emphasises the importance of 
connectivity in river systems, as fishes migrated in all conditions, 
but especially during rapidly-rising discharge.

2-Ecology of Freshwater Fish 2012: 21: 165-167. Do small tributaries 
function as refuges from floods? A test in a salmonid-dominated 
mountainous river
Itsuro Koizumi, Koh Hasegawa, Daisuke Kishi

1st 2 paragraphs:

Tributary streams are important for freshwater fishes, providing 
rearing, spawning and overwintering habitats (Northcote 1997). 
Tributaries may also function as temporal refuges from floods or 
droughts. Fisherman's anecdote often says that many large fishes were 
unexpectedly caught in small tributaries during floods, presumably 
because of immigrants from the main stem. Despite the plausibility, 
surprisingly few studies have examined the hypothesis, and limited 
observations rather showed weak evidence (Harvey et al. 1999; Han et 
al. 2007). As far as we know, a positive result was shown only by 
Nunn et al. (2010) in a floodplain tributary in a cyprinid-dominated 
lowland river.

On 8-10 August 2003, a powerful typhoon hit Hokkaido Island, Japan, 
accompanied with heavy rain, which allowed us to investigate the 
potential role of tributaries as refuges from flooding. We had just 
completed annual population census in four small tributaries of a 
river system 1-2 days before the typhoon. Thus, we planned additional 
census in the same manner in the four tributaries on 13-14 August 
(i.e., 3-4 days after the typhoon). This river system is ideal to 
examine fish movements between tributaries and the main stem because 
immigrants can be distinguished because of large differences in fish 
species and size structures between the two habitats.

Results:

Overall, our results did not support the hypothesis that many large 
fishes immigrate to small tributaries during floods. Radio-telemetry 
studies revealed that stream fishes can have remarkable tolerance to 
catastrophic floods by using micro-refuges, such as large woody 
debris, boulders or stream margins (Harvey et al. 1999; Makiguchi et 
al. 2009).

Alternatively, fishes may use tributaries only in the situations 
where not enough micro-refuges in the main stem are available. Given 
that the frequency of severe floods may be increasing worldwide 
because of climate change (Groisman et al. 2005), understanding 
conditions under which fishes escape into small tributaries during 
floods would become more important.

We, however, cannot exclude the possibility that the time scale 
fishes reacted the disturbance may be more rapid compared with our 
sampling period. If fishes used tributaries just during the peak flow 
and immediately went back to the main stem, our survey cannot reveal 
the escaping behaviour. Although our pre- and postflood comparison 
was conducted exceptionally within a narrow time-frame, even this may 
not be enough to detect the escapement from the main stem. Although 
it is dangerous and logically difficult, surveys during flooding 
should be favourable.
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