Variations in standard metabolic rate (SMR) are a potentially powerful explanation of macro-ecological processes. Better understanding of how SMR varies between species and over environmental gradients will enable more realistic predictions of processes at local levels. Using closed box respirometry, we measured SMR and factorial metabolic scope (MS) in two fish, the brown mudfish (Neochanna apoda), and the banded kokopu (BK, Galaxias fasciatus), to determine if their metabolism could explain the asymmetric distribution of these species across gradients of acidity, oxygen, flow and ephemerality. A mark-recapture study of 661 mudfish in 52 pools was also conducted to assess how mudfish populations and life-history traits compare with their metabolism.
Results/Conclusions
Mudfish were found in extremely acidic (averaging pH 4.31), poorly oxygenated (averaging 1.46 mg O2/L) pools that dried unpredictably. Mudfish were never found in streams or pools containing BK. In contrast, BK were found only in permanent pools and streams with higher pH, (averaging pH 4.91), and oxygen (averaging 2.16 mg O2/L). The tolerance of mudfish to extreme environmental conditions may be a strategy to avoid competition with, and predation by, BK. Despite their restriction to harsh environments, mudfish attained extremely high densities averaging 69 fish/m3, and reaching densities up to 260 fish/m3. This contrasted strongly to densities of BK, which averaged 4.45 fish/m3. Mudfish growth rates were low with many fish lacking length increases over 4 months. This was consistent with extremely low mudfish SMR’s, which averaged only 2.28 µmol O2/g/h and decreased with mass at a rate of –0.39 µmol O2/g/h. Mudfish MS averaged only 1.68 and increased with size by a factor of 0.05/g. These SMR and MS values are some of the lowest measured in fish. BK’s displayed higher SMR and MS in line with their lower densities and more oxygenated stream habitats. Low metabolism may be an adaptation for individual mudfish survival in their challenging environments, but may simultaneously increase their vulnerability to more powerful fish in benign habitats. Nevertheless, their metabolic profile may also allow their high densities as energy would be partitioned over more individuals. Thus mudfish yield powerful populations by being powerless individuals in poor environments.