Optimal ship routing of Tuna pen towing

The Problem

Southern Bluefin Tuna (SBT) are caught as juveniles during December to March in a region that extends from the mid-Great Australian Bight (GAB) to Kangaroo Island. The fish are then enclosed in very large pens and towed back to Port Lincoln at speeds less than than 1 knot (0.5 m/s) relative to the ocean currents so as to avoid bruising. The passage times can exceed 10 days.

The Solution

If the vessels and pens can “surf” the ocean currents then possibly there are routes that save time, fuel and money. In fact using the now-cast/forecasts, a mathematically optimal shortest route in time (and distance) can be determined. The optimal route must also assume some shallowest allowed depth (here 40 m) so as not to run aground.

An Example

An optimal solution was considered using the SAROM surface currents for the 5th to 15th January 2016, beginning at 34oS, 134oE in the eastern GAB (Figure 1). The solutions assume an initial guess of 4 waypoints that a skipper might select as the shortest way to return from the GAB to Port Lincoln. The results are summarised in Figure 1 displaying the:

  • fastest time route - shown in green - taking 199 hrs with a distance of 382 km
  • shortest distance route - shown in red - taking 239 hrs with a distance of 271 km.

The green fastest route is much longer in distance but takes advantage of offshore currents that help move the green "vessel" offshore and then to the south-east. The inshore red "vessel" has opposing currents that slow it down leading to an extra 40 hours of transit time.

View the effects on the shipping routes in animation (MP4 2.3 MB).

Figure 1: The optimal shortest distance route is shown in red, the fasted time route is shown in green. Both routes show an initial position in the GAB to the mouth of the Gulf from 5th - 15th January 2016. The initial route guess is 4 waypoints. The light arrows denote currents at the end of the transit and the 200 m isobath is indicated.
Figure 1: The optimal shortest distance route is shown in red, the fasted time route is shown in green. Both routes show an initial position in the GAB to the mouth of the Gulf from 5th - 15th January 2016. The initial route guess is 4 waypoints. The light arrows denote currents at the end of the transit and the 200 m isobath is indicated.

These results are preliminary but suggest that optimal routes may reduce passage times by 20% but take no account of:

  1. better guesses of initial transit routes
  2. importance of winds and waves on vessel and pen speeds.

Much research has been done in this area and remains to be implemented into eSA-Marine with a focus on industry needs.

Disclaimer

This is a research and experimental product and no liability is assumed for the use of the eSA-Marine system or its products including now-casts and forecasts. In particular, this includes all conditions outlined in the general PIRSA disclaimer that can be accessed here or at the bottom of this page. Moreover, other sources of ocean now-cast/forecasts and or data may not align or be consistent with output from the eSA-Marine System which may be in error and/or out of date. Users should obtain their own independent advice. No liability is assumed for the eSA-Marine products presented here.

Page Last Reviewed: 11 Aug 2017
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