A new method to determine in situ growth rates of decapod shrimp: a case study with brown shrimp Crangon crangon

Abstract

Shrimps are economically and ecologically very important, yet a lack of ageing techniques and hence unknown growth rates often impairs analytical assessments and management. A new method for the determination of in situ growth rates of shrimps is presented, based on dry weight condition. Since this index oscillates from low values directly after moult to highest values prior to moult in constantly feeding shrimp, the lowest observed pre-moult condition followed by a moult was introduced as a reference value to separate growing and starving individuals in field data. Experiments with Crangon crangon confirmed that (1) post-moult condition varies in a narrow physiologically optimal range, regardless of recent growth increments, and (2) dry weight condition prior to moult is closely related to the subsequent length increment. The method was applied to estimate growth increments from in situ dry weight condition data of C. crangon. The new method can easily be applied to other related species, since the required data can be obtained from very simple short-term experiments.

Appendix

Equation 5 can be further simplified if the dry weight condition before moulting (C1_−E ) is introduced.

$$ C1_{ - E} = \frac{W1 - E1}{{a \times L1^{b} }} $$

(a1)

Now in Eq. 5, the fraction is expanded with L1^b and Eq. a1 is applied to simplify

$$ {\text{INC}} = \left( {L1^{b} \times \frac{W1 - E1}{{a \times L1^{b} }}} \right)^{\frac{1}{b}} - \,L1 = \left( {L1^{b} \times C1_{ - E} **} \right)^{\frac{1}{b}} - \,L1\quad {\text{or}} $$

(a2)

$$ {\text{INC}} = L1 \times C1_{ - E}^{\frac{1}{b}} - L1 $$

(a3)

Equation a3 was used to describe the laboratory-based relationship between pre-moult condition C1_−E and the subsequently realised increment (INC). Here, a condition of C1_−E  = 1 corresponds to an increment of INC = 0.