A hunter peering through a high-powered scope at a bighorn sheep in the mountainous area outside Spences Bridge believed the ram was eight years old.

Kenneth Eng shot and killed the prized big-game species on Oct. 20, 2008 - the last day of the season - and took it to an Okanagan taxidermist.

But provincial authorities, acting on a tip from locals, thought the animal was only seven years old - and therefore, under provincial law, safe from hunters' bullets. A conservation officer seized the dead ram and a regional biologist determined it was only seven years old and not legal.

Eng filed an appeal in B.C. Supreme Court seeking to overturn his conviction under the Wildlife Act. He was originally ticketed and fined for killing an underage animal in 2008. Eng challenged that ticket at provincial court in 2011, but the conviction was upheld.

A Crown and defence lawyer argued merits of case law Thursday, interspersed with Californian bighorn sheep biology and identification.

"I've learned a lot about sheep hunting in reading this case," said Crown prosecutor John Blackman. "There's a different standard required of a sheep hunter."

Eng's lawyer, George Wool, told the court the conservation officer was not within his rights to seize the ram's head from a taxidermist to have its age determined.

"There was a tactical error made," he said.

Wool argued that seizure undermined the normal course of events required by the act, as Eng hadn't yet submitted the head - and its horns used to determine age - to a special inspector designated under the law within the 30 days he was allowed. The horns were never given to a special inspector but were instead viewed by other provincial authorities.

"He (conservation officer) didn't take it to the compulsory inspector because he knew, or ought to have known, it would be assessed as a legal ram," Wool told B.C. Supreme Court justice Kenneth Ball.

Blackman argued, however, that bighorn sheep hunting requires special knowledge and care that Eng didn't exercise when he came to Spences Bridge on the last day of the hunt after being skunked in the Kootenays.

Hunting regulations decree that a mature ram's horns must curl fully, so the tip is past the bridge of its nose. But those tips may break off, called "brooming," making identification more difficult. Another method involves counting rings, or annuli.

"This isn't shooting a grouse at the side of a logging road," Blackman said. "It requires expertise and patience."

Justice Ball reserved his decision to another date.

Identification is so complex that a North American expert and University of Calgary professor, Valerius Geist, first told Eng the animal was eight years after he was emailed a photo. But after provincial authorities brought him the actual horns, Geist changed his opinion to say the sheep was only seven.