Sexing accuracy and indicators of maturation status in captive Murray cod Maccullochella peelii peelii using non-invasive ultrasonic imagery

Abstract

Macroscopic- and histological-based assessments of gonad condition were compared with ultrasound images to determine the feasibility of this technology as a non-invasive diagnostic tool for identifying sex and assessing maturation status of Murray cod. Four age-classes (1+, 2+, 3+ and 6+ years), were sub-sampled at monthly intervals throughout their annual reproductive cycle and scanned with a 5 MHz linear transducer. An interpretation of sex was made from the resulting images and maximum cross-sectional gonad diameter and area were recorded. Fish were subsequently dissected to confirm gender, and the weights and maturation status of gonads determined and then compared with their respective image profile. Ovaries of females were usually a distinctive feature in ultrasound images, being particularly obvious in older and/or more developed fish. In contrast, the identification of male testis was more problematic. Nonetheless, identifying sex from ultrasound images was consistently achieved by recording the presence/absence of a female ovary (96% total sexing accuracy). Maximum cross-sectional ovary diameter and area were highly correlated with gonad weight (r² = 0.90 and 0.89, respectively) suggesting that indices of maturation status, comparable to the gonadosomatic index (GSI), can be obtained non-destructively from ultrasound scans of females. A less distinct relationship occurred between these dimensions and weight of testes (r² = 0.41). Significant increases (P < 0.05) in mean gonad index (GI, calculated from gonad diameter) occurred for most gonad development stages. However, differences in mean GI between maturation stages were confounded by phenotypic variability, indicating that GI may be limited to population level studies. Nevertheless, ultrasound images of ovaries at each development stage were visually distinctive and enabled qualitative evaluations of maturity, thereby complementing quantitative GI assessments. Repeated serial-monitoring of the same population using ultrasound appears to have great potential for tracking maturation-induced changes in broodfish.

Introduction

Non-lethal sex identification and evaluations of reproductive condition in finfish are crucial towards optimising aquaculture breeding protocols and the effective management of wild populations. However, for sexually monomorphic species, assessing the reproductive status of individuals typically relies on invasive methods that may compromise fish health and subsequent gamete quality. Murray cod (Maccullochella peelii peelii) are a large Australian freshwater fish with no apparent sexual-dimorphism (excluding abdominal swelling in gravid females) and a distinct annual reproductive cycle (Gooley et al., 1995, Rowland, 1998). Wild populations of this species have diminished dramatically during the previous century and consequently, more reliable assessments of the spawning and recruitment dynamics of Murray cod are required (Kearney and Kildea, 2001). Furthermore, due to an emerging aquaculture industry for Murray cod (Ingram et al., 2005), the development of effective management protocols for captive breeding programs are becoming of greater importance as more hatcheries become established. Nevertheless, reliable methods for accurately differentiating sex in this species do not currently exist outside of their spawning season, while evaluations of maturation status are presently limited to urogenital catherisation techniques (Ingram and Larkin, 2000) that elevate the risk of internal damage and/or pathogen introduction. Therefore, a means to reliably assess reproductive issues in Murray cod without impacting on the health of fish would be invaluable for scientific and management purposes.

The use of ultrasound as a diagnostic tool for assessing and monitoring aspects of fish reproduction is becoming increasingly popular as an alternative to other, more invasive methods such as blood biochemical analysis (e.g., Webb et al., 2002, Feist et al., 2004, Pottinger et al., 2005, Kucherka and Khan, 2007), endoscopy (e.g., Kynard and Kieffer, 2002, Wildhaber et al., 2005, Swenson et al., 2007) and urogenital catherisation. Several studies have already demonstrated the effectiveness of ultrasound technology for identifying sex and assessing reproductive condition of various fish species (Table 1). Nevertheless, even though the results from these studies have demonstrated the efficacy of ultrasound for reproductive diagnostic purposes, only limited data are available that compares fish of different age classes and/or fish at each stage of their reproductive development. Furthermore, ultrasound-based evaluations of maturation status have rarely been corroborated with more detailed assessments of gonad condition.

The present investigation employed macroscopic- and histological-based assessments of gonad condition to examine the effectiveness of ultrasound as a non-invasive diagnostic tool for identifying sex and evaluating reproductive status of captive Murray cod. The objectives of this study are to i) assess the accuracy of ultrasonography for identifying the sex of four age classes of Murray cod, and ii) obtain a detailed understanding of the ultrasound imaging characteristics, both quantitative and qualitative, of ovarian and testicular tissue at different stages of development by examining their correlation with more detailed assessments of gonad condition.