Projects

Use of otolith microchemistry of spotted seatrout to identify stock source-areas

End Date: 01/31/06

Abstract

The microchemistry of fish otoliths has been shown to be extremely useful as a biological tag. This microchemical “fingerprint” has been used as an environmental recorder to address various difficult fishery recruitment issues, including stock identification, the determination of migration pathways, the reconstruction of previous habitat information, age validation, and especially, for use as a natural tag of ambient conditions experienced during various life-history phases. Otoliths are already formed in newly-hatched fish larvae and continue to grow through concentric additions of alternating calcium carbonate and protein layers around a central nucleus. Also incorporated into the crystalline component of the otolith matrix are various trace elements, and the relative abundance of these elements in the otoliths is influenced by the chemical composition of the water in which the fish are growing.

Our proposed project is strengthened by findings from our ongoing Sea Grant study (extended until September 2003) that shows that juvenile spotted seatrout that inhabit different regions along the Mississippi coast can be distinguished with a high degree of accuracy by patterns of otolith microchemistry. Juveniles (n=199) were collected at nine locations across the Mississippi coast in 2001, extending from Grand Bay, Alabama to the Louisiana marshes east of the Mississippi River. Based on otolith microchemistry “fingerprints”, these fish could be classified with respect to their collection location with over 90% accuracy, and for several locations juveniles were classified with 100% accuracy.

The specific objectives of our proposed study are to: 1) During summer 2004 collect 24 age 3+ spotted seatrout from each of the same nine coastal regions where juvenile fish were collected in 2001 for otolith microchemistry analyses; 2) remove sagittal otoliths and analyze the inner portion (i.e., the portion that formed during the larval and young juvenile period in 2001) to identify stock-source areas and population movements (patterns of otolith microchemistry will be compared to regional patterns found in juveniles collected in 2001); and 3) collect juvenile spotted seatrout from the same nine coastal regions where we collected juvenile fish in 2001 and compare regional patterns in otolith microchemistry between 2001 and 2005 cohorts of spotted seatrout to determine if particular chemical “fingerprints” are typical for our different estuarine areas, or if there is much inter-annual variability.

We are currently analyzing the central portion of otoliths from one-year-old spotted seatrout collected in 2002 (i.e. the same year class as the juveniles collected in 2001). We expect, based on limited tag recapture information, that most of these fish will have originated from the same region in which they developed as juveniles. By analyzing the microchemistry of the central portion of otoliths from three-year-old spotted seatrout collected in 2004 (i.e. the same year class as the juveniles collected in 2001) we will reveal in which coastal regions these older fish grew up as juveniles. These older fish represent a critical reproductive period for this species, when movements are more likely to occur. This portion of our study will provide both stock source-areas, and information on population movements.

Juveniles will be collected from marsh-edge habitats with a 50’ bag seine, and adults will be collected with a gill net. Juvenile otoliths and the inner portion of adult otoliths (cleaned and dried) will be weighed, and then using a very precise laboratory protocol, the elemental composition of otoliths dissolved in 0.1 N re-distilled nitric acid will be determined using inductively coupled plasma mass spectrometry. The molar concentration of elements will be expressed as ratios to the molar concentration of Ca. In addition, stable isotope ratios of carbon and oxygen will be determined using an isotope ratio mass spectrometer. Principal Components Analysis will be used to reduce the otolith microchemical variables into multivariate composite variates for easy visual comparison of the regional ‘fingerprint’ patterns between 2001 and 2005. Multivariate Analysis of Variance will also be performed with individual otolith variables to compare statistical differences in microchemistry signatures as a means to identify distinct groups of locations for the 2005 data, and as a means to compare regional signatures between 2001 and 2005 regional patterns. Otolith weight will be included as a covariate to factor out ontogenetic variability in elemental composition. Canonical Discriminant Function Analysis will also be used to identify probable source areas of the reproductive adults. In addition, Neural Network Analysis will be used to classify individual adults with respect to their most probable source areas.

Spotted seatrout spend their entire lives in inshore coastal habitats, and as coastal development increases this species is consequently affected by both increased fishing pressure and habitat degradation. Information that can help with the management of this species will be extremely beneficial for resource managers, and techniques that we are developing can likely be applied to many coastal species.

Objectives

  1. to collect 24 juvenile spotted seatrout from the same nine coastal regions where we collected juvenile fish in 2001.
  2. to remove sagittal otoliths and analyze for spatial patterns in otolith microchemistry.
  3. to compare regional patterns in otolith microchemistry between 2001 and 2005 cohorts of spotted seatrout to determine if particular chemical ‘Afingerprints’ are typical for our different estuarine areas, or if there is much inter-annual variability. 

Methodology

Juveniles will be collected with a 50= bag seine, stored on ice, returned to the lab, and frozen until the otoliths are removed. Otoliths (cleaned and dried) will be weighed and then, using a very precise laboratory protocol, the elemental composition of otoliths dissolved in 0.1 N re-distilled nitric acid will be determined using inductively coupled plasma mass spectrometry. The molar concentration of elements will be expressed as ratios to the molar concentration of Ca. In addition, stable isotope ratios of carbon and oxygen will be determined using an Optima isotope ratio mass spectrometer. As with the analysis of otoliths from age 3 adults during year one, Principal Components Analysis will be used to reduce the otolith microchemical variables into multivariate composite variates for easy visual comparison of the regional fingerprint patterns between 2001 and 2005. Multivariate Analysis of Variance will also be performed with individual otolith variables to compare statistical differences in microchemistry signatures as a means to identify distinct groups of locations for the 2005 data and as a means to compare regional signatures between 2001 and 2005 regional patterns. Otolith weight will be included as a covariate to factor out ontogenetic variability in elemental composition. Canonical Discriminant Function Analysis will also be used to to classify the juveniles into multiple a priori regional groups, and Neural Network Analysis will be used to classify individuals with respect to their most probable source areas. 

Rationale

Based on our initial results, we know that otolith microchemistry fingerprints can effectively distinguish juveniles originating from the nine a priori coastal regions sampled in 2001. However, we do not know if these chemical ‘Afingerprints’ are typical for our different estuarine areas or if there is much inter-annual variability. This would need to be known for possible future studies of not only spotted seatrout, but also for other species that assuredly will be studied using these techniques. The results of this study of juvenile and age 3 spotted seatrout will greatly increase our understanding of both stock-source areas and population movements for this species. Spotted seatrout spend their entire lives in inshore coastal habitats, and as coastal development increases, this species is consequently affected by both increased fishing pressure and habitat degradation. Information that can help with the management of this species will be extremely beneficial for resource managers and techniques that we are developing can likely be applied to many coastal species.