Larval trawling techniques and analyses (hypothesis 5)
To aid in interpretation of otolith trace element signatures and infer
larval behaviour during their pelagic development, larval trawls were
conducted between 9–16 Jan 2016, aligning with the Sep –Nov peak
spawning period of landlocked G. brevipinnis (McDowall, 1990).
This time period ensured that both newly-hatched larvae and older
cohorts were present, providing insight into where larvae were located
for the duration of their entire pelagic development period. Night time
trawls were conducted given that nocturnal migrations of larvae into
surface waters commonly occur (Taylor, Graynoth, & James, 2000). Larval
habitat use in the lakes was assessed in three habitats: river plumes,
nearshore, and offshore. River plume sites were located near river
mouths in the turbid water outflowing from four tributaries from each
lake, including the two largest tributaries (Fig. 1; Wakatipu: Rees
River, Dart River; Wanaka: Makarora River, Matukituki River) and two
smaller tributaries (Wakatipu: Greenstone River, Buckler Burn; Wanaka:
Albert Burn, Boundary Creek). Nearshore sites were located <
30 m from the shoreline and > 100 m from other tributaries.
Offshore sites were > 200 m from the shore. Sampling was
conducted in a hierarchical design where each tributary was sampled as a
set of plume, nearshore and offshore sites.
Larvae were sampled using an ichthyoplankton tow net (125 x 50 cm, 250
µm mesh) with a flow meter (Ocean Test Equipment, Inc.;
www.oceantestequip.com) mounted in the mouth of the trawl. Tows lasted 4
mins at approximately 4 kmh-1. Plumes were often
relatively small, so were trawled in a circular motion to remain within
them. Trawl contents were immediately placed in 5% formalin. In the
laboratory, larvae were separated from organic and inorganic material,
and identified to species. To confirm species identification, a
subsample of larvae from each trawl was preserved in 90% ethanol in the
field, and the identity of 10 randomly selected larvae per trawl was
genetically confirmed using BLAST searches (Altschul, Gish, Miller,
Myers, & Lipman, 1990) of cytochrome b, amplified following the methods
of Waters, Craw, Youngson, and Wallis (2001). Gobiomorphus
cotidianus was the only other species likely to be encountered in these
lakes, but is morphologically distinct from G. brevipinnis(Jarvis & Closs, 2015). The density of larvae was determined by
dividing total number captured by the volume of water filtered (as
determined by the net measurements, flow meter, and trawl time),
resulting in a standardized estimate of the number of larvae per
m3 of water.
A G -test was used to assess differences in larval distribution
differences across offshore, near shore and tributary habitats of each
tributary. A one-tailed test was employed as larval densities are
predicted to be higher in plumes than other sampling locations.