Habitat Model

The spatial analysis tool HABITAT (Haasnoot & van de Wolfshaar, 2009; habitat.deltares.nl) was used (version 3.0.1.39291) to establish habitat suitability models for feeding and spawning adults and general requirements for survival of eggs, descending larvae and ascending juveniles. HABITAT uses response curves based on a set of critical habitat requirements as input. These response curves indicate the boundaries of environmental conditions for the expected presence of species. HABITAT translates each environmental parameter map to a habitat suitability index map on a scale from zero to one. The limiting

environmental parameter determines the eventual suitability, which is calculated as the minimum suitability of all input parameters per grid-cell. Suitable spawning locations determine the locations for the eggs, so therefore the conditions for egg incubation are tracked in cells that were suitable for spawning. For the other parameters, we assume that fish are able to swim towards other suitable sites (if they are available).

The habitat model has a similar structure to the habitat model described in Van Oorschot (2018). The main difference is the input from the hydro-morphological simulations, from which the values for the habitat input maps are derived. The model contains several life-stages and life-history events that take place at different moments in time (Figure 1.1 and Table 1.1). An overview of the calculated statistics per life stage, per input parameters and the response curves are presented in Annex C and D). Due to the use of the Morfac in the hydro-morphological model, the extraction of ecological input parameters at a specific time had to be adjusted to the new morphological time scales. This is different for the hydro-morphological model and the water quality model. Temperature values are now extracted from the water quality model for the years 2005-2007 instead of the more detailed measured values from 2014. This is done to match the calculated water quality parameters to the corresponding temperature. Also, the water quality parameters are not aggregated on a weekly basis, but the statistics are calculated directly from the raw water quality results.

Table 1.1    Life history events of P. altivelis

Figure 1.1 Life cycle of P. altivelis with timing of life stages.

For this study, maximum concentrations of suspended sediment were calculated based on the sum of 2 cohesive sediment fractions. This differs from the study by Van Oorschot (2018), where suspended sediment concentrations during the flushing were derived as maximum values from short-term simulations from Omer et al, (2017) and suspended sediment concentrations for the non-flushing periods were derived as mean values during the low-flow season.

1.1      Results

Model results could only be derived from the year 2005, before the instability occurred in the hydro-morphological model. This allows for the analysis of the results for spawning, which is the only life-event that occurs completely in 2005 (October – November).

 Figure 1.2 Habitat suitability results of spawning for this study for 2005. a) Reference reservoir water levels (50.6), b) reservoir water level of 47 and c) reservoir water level of 48.5.

Results show that the differences between the scenarios are relatively small (Figure 1.2 and Table 1.2). In all scenarios there is a larger suitable spawning area +/- 1.5 to 2.5 km kilometres downstream of the Funagira dam. Also, smaller patches of spawning area, albeit with a lower quality, are scattered all across the river. Results from the previous study, without morphological changes, show a smoother pattern with spawning sites more evenly spread across the river and with good spawning sites more upstream and downstream of the good spawning sites in the current study (Figure 1.3). Also, including morphological changes in the model apparently creates less suitable spawning area (Table 1.2).

Figure 1.3 Habitat suitability results of spawning of the previous for 2005. a) Reference reservoir water levels (50.6), b) reservoir water level of 47 and c) reservoir water level of 48.5.

Table 1.2    Habitat suitability statistics expressed in hectares for spawning area in 2005 of the current study and the previous study.

 1.2      Conclusion and discussion

 The new habitat suitability results show that there is less suitable spawning area when morphological changes are taken into account. Also the pattern is more scattered instead of smoothly distributed across the river. To investigate the reason for this reduction, the values of the input parameters have to be compared between the studies and the limiting factors have to be calculated. Moreover, these results have to be refined with new stable long-term hydro-morphological calculations and the inclusion of the other life-events (egg incubation, descending larvae, ascending larvae and feeding adults).